The most important commercial route to ester and acid manufacture is the liquid-phase oxidation of p-xylcne. However, if no special precautions are taken, the oxidation stops a tp-toluic acid:

Several methods are available to convert the second methyl group into a carboxyl group. A differentiation can be made between three basic possibilities:
I. The carboxyl group of p-toluic acid is esterified with methanol in a separate step (Witten, California Research) or, if methanol is used as solvent for the oxidation, esterification occurs simultaneously (BASF, Montccatini, DuPont); the second rnethyl group is then oxidized.
2. In addition to the metal salt catalyst (Mn or Co), a cocatalyst or promoter such as a bromine compound is used (Amoco/Mid-Century and IFP).
3. In a co-oxidation process, an auxiliary substance which is capable of supplying hydroperoxides is simultaneously oxidized. dehyde (Eastman Kodak), paraldehyde (Toray Industries), and methyl ethyl ketone (Mobil Oil and Olin Mathieson) are all used as co-oxidizable substances.

The first method yields, of course, dimethyl terephthalate, while recently the two others have been used mainly to manufacture the pure acid.

The Witten Drocess for the manufacture of dimethyltereDhtha-late, also known as the Imhausen or Katzschmann process, was developed at about the same time by California Research in 1950/51. Hercules contributed to the industrial development of the process with a California license and Irnhausen expertise. After taking over Chemischen Werke Witten, Dynamit Nobel (now Hüls) has become the leading European producer of DMT. In 1993, world production capacity for this process was about 3×10⁶ tonnes per year.

Both processes are two-step liquid-phase oxidations which produce p-toluic acid in the first step with air at 140-170 °C and 4-8 bar in the presence of Co/Mn salts of organic acids. A small amount of terephthalic acid is also formed. After esterification with methanol, e. g., catalyst-free at 250-280°C and almost 100 bar, or at 140-240 °C and up to 40 bar in the presence of a proton catalyst such as p-toluenesulfonic acid, the second methyl group can be oxidized to the mono of terephthalic acid. Industrially, the oxidation and esterification steps are combined. A mixture of p-xylene and p-toluic acid ester is oxidized and the products, toluic acid plus terephthalic acid monomethyl ester, are esterified together:

The crude esters are separated into their components in a system of columns under vacuum. The p-toluic acid ester is recycled to the oxidation.

The dimethyl terephthalate is further purified by recrystallizing twice from methanol or xylene, melted, and converted into readily manageable flakes with a drum flaker. The dimethyl terephthalate selectivity is about 85 % (based on p-xylene) and roughly 80% (based on CH₃OH).

Since then, Dynamit Nobel has developed their own process for the manufacture of fiber grade terephthalic acid so that the existing DMTplant could be converted to the production of TPA. BASF, Du Pont, and Montecatini combine the air oxidation of p-xylene in the liquid phase and the esterification with methanol in a single-step process:

In a countercurrent reactor p-xylene and recycled, partially oxidized product are introduced at the top, and methanol and air at the bottom. The oxidation is done with Co salts at 100-200°C and 5-20 bar, and a residence time of 22 hours. The crude dimethyl terephthalate selectivity is said to be greater than 90% (based on p-xylene) and 60-70% (based on methanol).

Older processes for oxidizing p-xylene with HNO₃ (BASF, Bergwerksverband Du Pont, Hoechst, ICI, Richfield) are no longer used commercially, since nitrogen-containing impurities are difficult to remove from terephthalic acid.