Maleic acid 30%, C4H4O4, is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups.

It is employed in the manufacture of phthalic-type alkyd and polyester resins, surface coatings, copolymers, plasticizers, lubricant additives and agricultural chemicals; in adhesives and sealants; as a preservative for oils and fats; organic synthesis; dyeing and finishing wool, cotton and silk; and preparing the maleate salts of antihistamines and similar drugs.