(CAS NO. ) is colourless, odourless, very hygroscopic liquid, synthesised from dimethyl sulfide. The main impurity is water, with a trace of dimethyl sulfone. The Karl-Fischer test is applicable. It is dried with Linde types 4A or 13X molecular sieves, by prolonged contact and passage through a column of the material, then distd under reduced pressure. Other drying agents include CaH₂, CaO, BaO and CaSO₄. It can also be fractionally crystd by partial freezing. More extensive purification is achieved by standing overnight with freshly heated and cooled chromatographic grade alumina. It is then refluxed for 4h over CaO, dried over CaH₂, and then fractionally distd at low pressure. For efficiency of desiccants in drying dimethyl sulfoxide.

Rapid purification: Stand over freshly activated alumina, BaO or CaSO₄ overnight. Filter and distil over CaH₂ under reduced pressure (~ 12 mm Hg). Store over 4A molecular sieves.