Maleic acid, unsaturated organic dibasic acid, is used in making polyesters for fibre-reinforced laminated moldings and paint vehicles, and in the manufacture of fumaric acid and numerous additional chemical products. Maleic acid and its anhydride are prepared industrially by the catalytic oxidation of benzene.

Maleic acid shows reactions typical of both olefins and carboxylic acids. Commercially significant reactions of the acid groups include esterification with glycols to polyesters and dehydration to the anhydride. The double bond is involved in conversions to fumaric acid, to sulfosuccinic acid (used in wetting agents), and to Malathion (an insecticide).

Maleic acid is an essential raw material used in the manufacture of lubricant additives, unsaturated polyester resins, surface coatings, plasticizers, copolymers and agricultural chemicals.