75-60-5

Basic information

• Product Name: CACODYLIC ACID
• Synonyms: Cacodylic acid >=99.0%;ANSAR(R);CACODYLIC ACID;Cacodylie acid;HYDROXYDIMETHYLARSINE OXIDE;DIMETHYLHYDROXYARSINE OXIDE;DIMETHYLARSINIC ACID;DIMETHYLARSONIC ACID
• CAS NO: 75-60-5
• Molecular Formula: C₂H₇AsO₂
• Molecular Weight: 138
• EINECS: 200-883-4
• Product Categories: As (Arsenic) Compounds;Classes of Metal Compounds;Semimetal Compounds;organoarsenic compound;Aliphatics;Intermediates & Fine Chemicals;Pharmaceuticals;buffer
• Mol File: 75-60-5.mol
• Article Data: 8

Chemical Properties

• Melting Point: 195-198 °C
• Boiling Point: 253.1 °C at 760 mmHg
• Flash Point: 109.4 °C
• Appearance: White/Powder/Solid
• Vapor Pressure: 0.00288mmHg at 25°C
• Storage Temp.: Store at RT.
• PKA: 1.57(at 25℃)
• Water Solubility: Soluble in water (50 mg/ml), water (2000 at 25°C), alcohols, and acetic acid. Insoluble in ether.
• Sensitive: Hygroscopic
• Stability: Aqueous solutions react violently with active metals. Incompatible with strong oxidizing agents, strong bases.
• Merck: 14,1604
• BRN: 1736965
• CAS DataBase Reference: CACODYLIC ACID(CAS DataBase Reference)
• NIST Chemistry Reference: CACODYLIC ACID(75-60-5)
• EPA Substance Registry System: CACODYLIC ACID(75-60-5)

Safety Data

• Hazard Codes: T,N
• Statements: 23/25-50/53
• Safety Statements: 20/21-28-45-60-61
• RIDADR: UN 1572 6.1/PG 2
• WGK Germany: 3
• RTECS: CH7525000
• F: 3-10
• TSCA: Yes
• HazardClass: 6.1
• PackingGroup: II
• Hazardous Substances Data: 75-60-5(Hazardous Substances Data)

Usage

Chemical Properties

white crystals or powder

Uses

Different sources of media describe the Uses of 75-60-5 differently. You can refer to the following data:
1. Cacodylic acid is used for dermatologic treatment in cronic eczema, anemia and as a tonic.
2. antieczema, dermatologic, herbicide
3. A useful arsenic acid for proteomics research. It a good substitute for phosphate in applications to avoid phosphates. Cacodylic Acid is useful for some DNA applications and is also popular in microscopy.

Definition

ChEBI: The organoarsenic compound that is arsenic acid substituted on the central arsenic atom with two methyl groups.

General Description

A colorless, odorless crystalline solid. Melting point 195-196°C. Toxic by ingestion and irritating to skin and eyes.

Air & Water Reactions

Hygroscopic. Water soluble.

Reactivity Profile

CACODYLIC ACID is a weak acid. Dissolves in water to yield solutions containing more hydrogen ions than pure water contains and so having a pH less than 7.0. Is neutralized exothermically by all bases to produce water plus a salt. Reacts (but usually slowly) with active metals to form gaseous hydrogen and a metal salt. Such reactions occur in principle for the solid acid but are quite slow if the solid acid remains dry. The solid may absorb enough water from the air and dissolve sufficiently in CACODYLIC ACID to corrode or dissolve iron, steel, and aluminum parts and containers. Reacts with cyanide salts to generate gaseous hydrogen cyanide. Flammable and/or toxic gases and heat may be generated with diazo compounds, dithiocarbamates, isocyanates, mercaptans, nitrides, and sulfides. Also may react with sulfites, nitrites, thiosulfates (to give H2S and SO3), dithionites (SO2), to generate flammable and/or toxic gases and heat. Reaction with carbonates and bicarbonates generates a harmless gas (carbon dioxide) but still some heat. Can be oxidized exothermically by strong oxidizing agents and reduced by strong reducing agents; a wide variety of products is possible. May initiate polymerization reactions; may catalyze (increase the rate of) chemical reactions.

Hazard

Toxic by ingestion.

Health Hazard

Chemical is essentially non-irritating in contact with skin or eyes. Ingestion causes arsenic poisoning, but symptoms are delayed.

Fire Hazard

Behavior in Fire: May form toxic oxides of arsenic when heated.

Shipping

UN1572 Cacodylic acid & UN1688 Sodium Cacodylate, Hazard Class: 6.1; Labels: 6.1-Poisonous materials. UN3465 Organoarsenic compound, solid, n.o.s., Hazard Class: 6.1; Labels: 6.1-Poisonous materials, Technical Name Required

Purification Methods

Recrystallise it from warm EtOH (3mL/g) by cooling and filtering. Dry it in a vacuum desiccator over CaCl2. It has also been recrystallised twice from propan-2-ol. [Koller & Hawkridge J Am Chem Soc 107 7412 1985, Beilstein 4 IV 3681.]

Incompatibilities

A strong reducing agent. Incompatible with oxidizers (chlorates, nitrates, peroxides, permanganates, perchlorates, chlorine, bromine, fluorine, etc.); contact may cause fires or explosions. Keep away from alkaline materials, strong bases, strong acids, oxoacids, epoxides. Aqueous solution reacts violently with chemically active metals releasing toxic arsenic fumes. Incompatible with oxidizers, sulfuric acid; caustics (strong bases), reducing agents; ammonia, amines, isocyanates, alkylene oxides; epichlorohydrin.

InChI:InChI=1/C2H7AsO2/c1-3(2,4)5/h1-2H3,(H,4,5)

Synthetic route

trimethylarsine (593-88-4) → trimethylarsine oxide (4964-14-1) + Dimethylarsinic acid (75-60-5)

Conditions	Yield
With air
With diethyl ether

Dimethylchloroarsine (557-89-1) → Dimethylarsinic acid (75-60-5)

Conditions	Yield
With dihydrogen peroxide

tetramethyldiarsine (471-35-2) → Dimethylarsinic acid (75-60-5)

Conditions	Yield
bei der Oxydation;

bis-{dimethylarsenic}-oxide (503-80-0) → Dimethylarsinic acid (75-60-5)

Conditions	Yield
Oxydiert sich an der Luft;
With air
With water; mercury(II) oxide
With water; oxygen

allyldimethylarsine (691-35-0) → Dimethylarsinic acid (75-60-5) + allyl-dimethyl-arsine oxide

Conditions	Yield
With diethyl ether

sodium dimethylarsenide (13787-40-1) → Dimethylarsinic acid (75-60-5)

Conditions	Yield
With oxygen In tetrahydrofuran

3,5-dinitrosalicylic acid (609-99-4) + cacodylic acid anion (15132-04-4) → Dimethylarsinic acid (75-60-5) + 3,5-dinitrosalicylic acid anion (52040-82-1)

Conditions	Yield
at 25℃; Rate constant;

methylarsonic Acid (124-58-3) + S-Adenosyl-L-methionine (14031-35-7, 17176-17-9, 29908-03-0, 75044-81-4, 78548-84-2, 79647-17-9, 91279-78-6) → Dimethylarsinic acid (75-60-5)

Conditions	Yield
With GLUTATHIONE; DL-dithiothreitol; magnesium chloride; monomethylarsonic acid methyltransferase from rabbit liver In various solvent(s) at 37℃; for 1.5h; Methylation; Enzymatic reaction;

S-Adenosyl-L-methionine (14031-35-7, 17176-17-9, 29908-03-0, 75044-81-4, 78548-84-2, 79647-17-9, 91279-78-6) → Dimethylarsinic acid (75-60-5) + methylarsonic Acid (124-58-3)

Conditions	Yield
With trisodium arsenite; GLUTATHIONE; magnesium chloride; arsenite methyltransferase from rabbit liver In various solvent(s) at 37℃; for 1h; Methylation; Enzymatic reaction;

'Cadet's liquid'(cacodyl oxide mainly) → Dimethylarsinic acid (75-60-5)

Conditions	Yield
With sodium hypochlorite; water; oxygen Reagens 4: Aceton;
With iron(III) oxide; water
With sulfuric acid Electrolysis.an Platinelektroden;

tetramethyldiarsine (471-35-2) + sulfuric acid (7664-93-9) → Dimethylarsinic acid (75-60-5)

bis-{dimethylarsenic}-oxide (503-80-0) + dihydrogen peroxide (7722-84-1) + acid → Dimethylarsinic acid (75-60-5)

Dimethylchloroarsine (557-89-1) + ammonium persulfate → Dimethylarsinic acid (75-60-5)

bis-{dimethylarsenic}-oxide (503-80-0) + water (7732-18-5) + dihydrogen peroxide (7722-84-1) + mercury oxide → Dimethylarsinic acid (75-60-5)

trichloro-dimethyl-arsorane (558-31-6) + water (7732-18-5) → hydrogenchloride (7647-01-0) + Dimethylarsinic acid (75-60-5)

tetramethyldiarsine (471-35-2) + air → bis-{dimethylarsenic}-oxide (503-80-0) + Dimethylarsinic acid (75-60-5)

dimethylarsine (593-57-7) + oxide of/the/ nitrogen → tetramethyldiarsine (471-35-2) + bis-{dimethylarsenic}-oxide (503-80-0) + Dimethylarsinic acid (75-60-5)

trimethylarsine (593-88-4) + air → trimethylarsine oxide (4964-14-1) + Dimethylarsinic acid (75-60-5)

dimethylarsine (593-57-7) + liquid sulfur dioxide → methyl-arsenic sulfide (2533-82-6) + trimethylarsine sulphide (26386-93-6) + dimethylarsino dimethyldithioarsinate (59840-68-5) + Dimethylarsinic acid (75-60-5)

diethyl ether (60-29-7) + allyldimethylarsine (691-35-0) + air → Dimethylarsinic acid (75-60-5) + allyl-dimethyl-arsine oxide

dimethylarsine (593-57-7) + sulfuric acid (7664-93-9) → cacodyl sulfide (591-10-6) + Dimethylarsinic acid (75-60-5) + dimethyl-arsine; sulfate

bis-{dimethylarsenic}-oxide (503-80-0) + dihydrogen peroxide (7722-84-1) + alkali → trimethylarsine (593-88-4) + methylarsine oxide (593-58-8) + Dimethylarsinic acid (75-60-5) + arsenous acid

Conditions	Yield
Produkt 5: Trimethylarsinoxyd; Produkt 6: Methylarsinsaeure;

bis-{dimethylarsenic}-oxide (503-80-0) + water (7732-18-5) + oxygen + alkali → trimethylarsine (593-88-4) + methylarsine oxide (593-58-8) + Dimethylarsinic acid (75-60-5) + arsenous acid

Conditions	Yield
Produkt 5: Trimethylarsinoxyd; Produkt 6: Methylarsinsaeure;

Dimethylchloroarsine (557-89-1) + water (7732-18-5) + 4,4'-azoxy anisole (1562-94-3, 21650-70-4, 51437-65-1) → hydrogenchloride (7647-01-0) + Dimethylarsinic acid (75-60-5) + mercury

Conditions	Yield
das erhaltene Produkt beim Kochen mit Wasser;

Dimethylchloroarsine (557-89-1) + water (7732-18-5) + mercury (II)-chloride → hydrogenchloride (7647-01-0) + Dimethylarsinic acid (75-60-5) + 4,4'-azoxy anisole (1562-94-3, 21650-70-4, 51437-65-1) + mercury

Conditions	Yield
das erhaltene Produkt zerfaellt beim Kochen mit Wasser;

dimethylarsinous acid (55094-22-9) → Dimethylarsinic acid (75-60-5)

Conditions	Yield
With oxygen In water-d2 at 24.3℃; pH=3.2; Kinetics; Further Variations:; pH-values;

dimethylarsine (593-57-7) + Nitrogen dioxide (10102-44-0) → cacodyloxide (418758-51-7) + Dimethylarsinic acid (75-60-5)

Conditions	Yield
byproducts: cacodyl; inflamation;
byproducts: cacodyl; inflamation;

methylcobalamin + arsenic(III) trioxide → monomethylarsinic acid + Dimethylarsinic acid (75-60-5)

Conditions	Yield
With GLUTATHIONE; selenious acid In water at 37℃; for 360h; pH=3; Product distribution / selectivity; Tris-HCl buffer solution;

monomethylarsinic acid + methylcobalamin → Dimethylarsinic acid (75-60-5)

Conditions	Yield
With GLUTATHIONE In water at 37℃; for 7h; pH=7.6; Product distribution / selectivity; Tris-HCl buffer solution;

bismuth (III) nitrate pentahydrate + Dimethylarsinic acid (75-60-5) → bis(dimethylarsinato)bismuth nitrate monohydrate

Conditions	Yield
In acetone at 20℃; for 3h;	100%
In acetone at 20℃; for 3h;	96%

Dimethylarsinic acid (75-60-5) + antimony(III) chloride (10025-91-9) → tris(dimethylarsinato)antimony

Conditions	Yield
In acetone at 20℃; for 5h;	99%

Dimethylarsinic acid (75-60-5) + palladium diacetate (3375-31-3) → bis(dimethylarsinato)palladium(II)

Conditions	Yield
In acetone Pd acetate added to suspn. of cacodylic acid in acetone; stirred vigoroously for 48 h; centrifuged, washed with acetone; crude product extd. with CH2Cl2 by centrifugation; extract combined, evapd., dried in vacuo; elem. anal.;	98%

aluminum (III) chloride (7446-70-0, 7784-13-6) + Dimethylarsinic acid (75-60-5) → tris(dimethylphosphinato)aluminum

Conditions	Yield
In acetone at 20℃; for 24h;	97%

iron(II) trifluoromethanesulfonate acetonitrile disolvate + Dimethylarsinic acid (75-60-5) + sodium tetraphenyl borate (143-66-8) + N,N,N′,N′-tetrakis((1-ethylbenzimidazol-2-yl)methyl)-2-hydroxy-1,3-diaminopropane (79724-80-4) → [Fe2(N,N,N',N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane(-1H))(μ-O2AsMe2)](BPh4)(OTf) (1178891-43-4)

Conditions	Yield
With (C2H5)3N In methanol HOCH(CH2N(CH2C7H4N2C2H5)2)2 dissolved in MeOH along with (C2H5)3N, (CH3)2AsO2H added, Fe salt added, NaB(C6H5)4 added after 5 min; filtered, dried (vac.), recrystd. (MeCN and Et2O); elem. anal.;	93%

gallium(III) trichloride + Dimethylarsinic acid (75-60-5) → tris(dimethylarsinato)gallium

Conditions	Yield
In methanol; diethyl ether at 20℃; for 2h;	91%

indium(III) chloride + Dimethylarsinic acid (75-60-5) → tris(dimethylphosphinato)indium

Conditions	Yield
Stage #1: indium(III) chloride; Dimethylarsinic acid In acetone at 20℃; for 1h; Stage #2: With triethylamine In acetone at 20℃; for 48h;	89%

Dimethylarsinic acid (75-60-5) → methylarsenic dibromide (676-70-0)

Conditions	Yield
With hydrogen bromide In water at 130℃; for 5h;	84%

bismuth(III) chloride + Dimethylarsinic acid (75-60-5) → tris(dimethylarsinato)bismuth

Conditions	Yield
In acetone at 20℃; for 5h;	80%

Dimethylarsinic acid (75-60-5) + bismuth(III) chloride (7787-60-2) → tris(dimethylarsinato)bismuth

Conditions	Yield
In acetone at 20℃; for 5h;	80%

Dimethylarsinic acid (75-60-5) → dimethyliodoarsine (676-75-5)

Conditions	Yield
With sulfuric acid; sulfur dioxide; potassium iodide In water for 3h;	76%
With sulfuric acid; sulfur dioxide; potassium iodide
With hydrogenchloride; sulfur dioxide; potassium iodide
Multistep reaction;
With sulfuric acid; sulfur dioxide; potassium iodide In water

Dimethylarsinic acid (75-60-5) + 18C2H3O2(1-)*8H2O*2H(1+)*Mn12O12(16+) → 6C2H6AsO2(1-)*Mn4O4(6+)

Conditions	Yield
In acetonitrile	75%

Dimethylarsinic acid (75-60-5) + zirconium(IV) tert-butoxide → Zr3(μ,μ'-O2AsMe2)2(μ2,μ'-O2AsMe2)(O-t-Bu)7(μ-O-t-Bu)2 (606948-98-5)

Conditions	Yield
In toluene under N2 atm. to Zr(O-t-Bu)4 in toluene was added cacodilic acid and heated with stirring at 70°C for 24 h; react. mixt. was cooled, solvent was removed in vac., residue was dissolved in hexane, filtered, and kept at -10°C; elem. anal.;	69%

Dimethylarsinic acid (75-60-5) + di(n-butyl)tin oxide (818-08-6) → Bu2Sn(O2AsMe2)2 C12H30As2O4Sn, orthorhombic

Conditions	Yield
In methanol Bu2SnO, HO2AsMe2 mixed in MeOH; refluxed for 3 h; filtered off; set aside at ambient temp. in open flask; filtered off after 1 d; set aside in refrigerator in closed flask; filtered off after 3 d; recrystd. from MeOH; elem. anal.;	64%

Dimethylarsinic acid (75-60-5) + dichlorodiethylstannane (866-55-7) → (CH3CH2)2ClSnO2As(CH3)2

Conditions	Yield
In methanol byproducts: HCl; 2 equiv. of As-compd. in MeOH was added to MeOH soln. of Sn-compd., keeping for 2 weeks at room temp.; soln. was concd. by partial evapn. of MeOH at room temp., crystals were filtered off, washed with small amt. of MeOH, dried in vac., elem. anal.;	42%

Dimethylarsinic acid (75-60-5) + dimethyltin dichloride (753-73-1) → (CH3)2ClSnO2As(CH3)2

Conditions	Yield
In methanol byproducts: HCl; 2 equiv. of As-compd. in MeOH was added to MeOH soln. of Sn-compd., keeping for 2 months at room temp.; soln. was concd. by partial evapn. of MeOH at room temp., crystals were filtered off, washed with small amt. of MeOH, dried in vac., elem. anal.;	33%

Dimethylarsinic acid (75-60-5) + 6C5H9O2(1-)*3H2O*Cl(1-)*Fe3O(7+) + acetonitrile (75-05-8) → 8C5H9O2(1-)*3H2O*3Cl(1-)*15C2H3N*[(17O2As(CH3)2)(Fe12O4)](11+)

Conditions	Yield
at 20℃;	31%

Dimethylarsinic acid (75-60-5) → Dimethylchloroarsine (557-89-1)

Conditions	Yield
With hydrogenchloride; tin(ll) chloride In water at 5 - 20℃; Cooling with ice; Inert atmosphere;	26%
With hydrogenchloride; sodium hypophosphite
With hydrogenchloride; tin(ll) chloride

methanol (67-56-1) + Iron(III) nitrate nonahydrate + 8-quinolinol (148-24-3) + Dimethylarsinic acid (75-60-5) → NO3(1-)*2C9H6NO(1-)*4CH4O*[(6O2As(CH3)2)(AsHO2(CH3)2)(Fe3)](3+)

Conditions	Yield
With triethylamine for 3h;	26%

strontium perchlorate monohydrate + Dimethylarsinic acid (75-60-5) + [Mn12O12(O2CPh)16(H2O)4] → 16Mn(3+)*4Sr(2+)*16C7H5O2(1-)*24C2H6AsO2(1-)*8O(2-)

Conditions	Yield
In methanol; acetonitrile for 0.333333h;	7%

calcium(II) nitrate tetrahydrate + Dimethylarsinic acid (75-60-5) + [Mn12O12(O2CPh)16(H2O)4] → 16Mn(3+)*4Ca(2+)*16C7H5O2(1-)*24C2H6AsO2(1-)*8O(2-)

Conditions	Yield
In methanol; acetonitrile for 0.333333h;	5%

Thioglycolamide (758-08-7) + Dimethylarsinic acid (75-60-5) → dimethylarsinomercapto-acetic acid amide (133401-89-5) + NSC 28727 (64057-55-2)

Conditions	Yield
With water

Dimethylarsinic acid (75-60-5) + l-cysteine hydrochloride (52-89-1) → cysteinyldimethylthioarsenite (91919-78-7)

Dimethylarsinic acid (75-60-5) → trichloro-dimethyl-arsorane (558-31-6)

Conditions	Yield
With diethyl ether; phosphorus pentachloride

Dimethylarsinic acid (75-60-5) → cacodyl sulfide (591-10-6)

Conditions	Yield
With hydrogen sulfide; water

Upstream product

• 503-80-0 bis-{dimethylarsenic}-oxide 
• 609-99-4 3,5-dinitrosalicylic acid 
• 15132-04-4 cacodylic acid anion 
• 7722-84-1 dihydrogen peroxide 
• 7732-18-5 water 
• 557-89-1 Dimethylchloroarsine 
• 1562-94-3 4,4'-azoxy anisole 
• 124-58-3 methylarsonic Acid 
• 593-57-7 dimethylarsine 
• 10102-44-0 Nitrogen dioxide 
• 55094-22-9 dimethylarsinous acid 
• 7664-93-9 sulfuric acid 
• 60-29-7 diethyl ether 
• 691-35-0 allyldimethylarsine 

Downstream Products

• 133401-89-5 dimethylarsinomercapto-acetic acid amide 
• 64057-55-2 NSC 28727 
• 558-31-6 trichloro-dimethyl-arsorane 
• 676-75-5 dimethyliodoarsine 
• 7732-18-5 water 
• 7553-56-2 iodine 
• 69819-86-9 Darinaparsin 
• 64436-13-1 arsenobetaine 
• 4964-14-1 trimethylarsine oxide 
• 1178891-43-4 [Fe2(N,N,N',N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane(-1H))(μ-O2AsMe2)](BPh4)(OTf) 
• 1337979-42-6 [Ph2Sn(O2AsMe2)(μ-OMe)]2 
• 676-70-0 methylarsenic dibromide 
• 91919-78-7 cysteinyldimethylthioarsenite 
• 13494-80-9 hydrogen telluride 

Relevant articles and documents

The rate of oxidation of dimethylarsinous acid to dimethylarsinic acid is pH dependent: Implications for the analysis and toxicology of arsenic metabolites in urine

A 1H NMR spectroscopic study has shown the rate oxidation of dimethylarsinous acid to dimethylarsinic acid in buffered aqueous solutions to depend upon pH. Dimethylarsinous acid has been reported to be a highly toxic arsenical metabolite and component of the urine of persons exposed to inorganic arsenic, particularly through drinking water. As the pH of human urine can range from 4.5 to 8, the pH dependence of the oxidation rate of dimethylarsinous acid to dimethylarsinic acid has profound implications for the detection and analysis of these compounds in urine samples, and for the relevance of the experimental toxicology of dimethylarsinous acid.

Methylated Phenylarsenical Metabolites Discovered in Chicken Liver

We report the discovery of three toxicologically relevant methylated phenylarsenical metabolites in the liver of chickens fed 3-nitro-4-hydroxyphenylarsonic acid (ROX), a feed additive in poultry production that is still in use in several countries. Methyl-3-nitro-4-hydroxyphenylarsonic acid (methyl-ROX), methyl-3-amino-4-hydroxyphenylarsonic acid (methyl-3-AHPAA), and methyl-3-acetamido-4-hydroxyphenylarsonic acid (or methyl-N-acetyl-ROX, methyl-N-AHPAA) were identified in such chicken livers, and the concentration of methyl-ROX was as high as 90 μg kg?1, even after a five-day clearance period. The formation of these newly discovered methylated metabolites from reactions involving trivalent phenylarsonous acid substrates, S-adenosylmethionine, and the arsenic (+3 oxidation state) methyltransferase enzyme As3MT suggests that these compounds are formed by addition of a methyl group to a trivalent phenylarsenical substrate in an enzymatic process. The IC50 values of the trivalent phenylarsenical compounds were 300–30 000 times lower than those of the pentavalent phenylarsenicals.

Enzymatic methylation of arsenic compounds: Assay, partial purification, and properties of arsenite methyltransferase and monomethylarsonic acid methyltransferase of rabbit liver

A rapid, accurate, in vitro assay utilizing radioactive S-adenosylmethionine (SAM) has been developed for the methylation of arsenite and monomethylarsonate (MMA) by rabbit liver methyltransferases. The assay has been validated by separating, identifying, and measuring the products of the reaction using chloroform extraction, ion exchange chromatography, TLC, or HPLC. The enzymes involved in this pathway, arsenite methyltransferase and MMA methyltransferase, have been purified approximately 2000-fold from rabbit liver. After gel electrophoresis, a single band is obtained with both enzyme activities in it. The pH optima for purified arsenite methyltransferase and monomethylarsonic acid methyltransferase are 8.2 and 8.0, respectively. A thiol, S-adenosylmethionine, and arsenite are required for the partially purified arsenite methyltransferase that catalyzes the synthesis of monomethylarsonate. A different enzyme activity that catalyzes the methylation of monomethylarsonate to dimethylarsinate also requires SAM and a thiol. Even though arsenite methyltransferase and monomethylarsonate methyltransferase have different substrates, pH optima, and saturation concentrations for their substrates, whether the two activities are present on one protein molecule or different protein molecules is still uncertain. Both activities have a molecular mass of 60 kDa as determined by gel exclusion chromatography. There is no evidence at the present time for these enzyme activities being on different protein molecules. Neither arsenate, selenate, selenite, or selenide are methylated by the purified enzyme preparations. Results from the use of crude extracts, often called cytosol, to study the properties of these methyltransferases dealing with arsenic species should be viewed with caution since such crude extracts contain inhibiting and other interfering activities.