Terephthalic acid is an organic compound with formula C₆H₄(CO₂H)₂. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tons are produced annually. The common name is derived from the turpentine-producing tree Pistacia terebinthus and phthalic acid.

Terephthalic acid is also used in the production of PBT plastic (polybutylene terephthalate).

Terephthalic acid was first isolated (from turpentine) by the French chemist Amédée Cailliot (1805–1884) in 1846. Terephthalic acid became industrially important after World War II. Terephthalic acid was produced by oxidation of p-xylene with 30-40% nitric acid. Air oxidation of p-xylene gives p-toluic acid, which resists further air-oxidation. Esterification of p-toluic acid to methyl p-toluate (CH₃C₆H₄CO₂CH₃) opens the way for further oxidation to monomethyl terephthalate. In the Dynamit−Nobel process these two oxidations and the esterification were performed in a single reactor. The reaction conditions also lead to a second esterification, producing dimethyl terephthalate, which could be hydrolysed to terephthalic acid. In 1955, Mid-Century Corporation and ICI announced the bromide-catalysed oxidation of p-toluic acid directly to terephthalic acid, without the need to isolate intermediates and still using air as the oxidant. Amoco (as Standard Oil of Indiana) purchased the Mid-Century/ICI technology, and the process is now known by their name.

In the Amoco process, which is widely adopted worldwide, terephthalic acid is produced by catalytic oxidation of p-xylene:

The process uses a cobalt–manganese–bromide catalyst. The bromide source can be sodium bromide, hydrogen bromide or tetrabromoethane. Bromine functions as a regenerative source of free radicals. Acetic acid is the solvent and compressed air serves as the oxidant. The combination of bromine and acetic acid is highly corrosive, requiring specialized reactors, such as those lined with titanium. A mixture of p-xylene, acetic acid, the catalyst system, and compressed air is fed to a reactor.

The oxidation of p-xylene proceeds by a free radical process. Bromine radicals decompose cobalt and manganese hydroperoxides. The resulting oxygen-based radicals abstract hydrogen from a methyl group, which have weaker C–H bonds than does the aromatic ring. Many intermediates have been isolated. p-xylene is converted to p-toluic acid, which is less reactive than the p-xylene owing to the influence of the electron-withdrawing carboxylic acid group. Incomplete oxidation produces 4-carboxybenzaldehyde (4-CBA), which is often a problematic impurity.

Approximately 5% of the acetic acid solvent is lost by decomposition or "burning". Product loss by decarboxylation to benzoic acid is common. The high temperature diminishes oxygen solubility in an already oxygen-starved system. Pure oxygen cannot be used in the traditional system due to hazards of flammable organic–O₂ mixtures. Atmospheric air can be used in its place, but once reacted needs to be purified of toxins and ozone depleters such as methylbromide before being released. Additionally, the corrosive nature of bromides at high temperatures requires the reaction be run in expensive titanium reactors.

The use of carbon dioxide overcomes many of the problems with the original industrial process. Because CO₂ is a better flame inhibitor than N₂, a CO₂ environment allows for the use of pure oxygen directly, instead of air, with reduced flammability hazards. The solubility of molecular oxygen in solution is also enhanced in the CO₂ environment. Because more oxygen is available to the system, supercritical carbon dioxide (T_c = 31 °C) has more complete oxidation with fewer byproducts, lower carbon monoxide production, less decarboxylation and higher purity than the commercial process.