fumaric acid (fyōmărˈĭk) [key] or trans -butenedioic acid, HO2CCH&eq;CHCO2H, unsaturated dicarboxylic acid that melts at 287°C. Maleic acid, or cis -butenedioic acid, is a geometric isomer of fumaric acid; it melts at about 140°C. Of the two isomers fumaric acid is the more stable and can be prepared from maleic acid by heating it. Fumaric acid can be prepared by catalytic oxidation of benzene or by bacterial action on glucose. It is found in small amounts in a variety of plants and is essential to the respiration of animal and vegetable tissue. Fumaric acid is used as a substitute for tartaric acid in beverages and baking powder. It is used as a mordant in dyeing and in the manufacture of synthetic resins and polyhydric alcohols.