Maleic anhydride is a suitable and easily available substrate for preparation of organic acids for pharmaceutical and food industries. The paper describes preparation, based on maleic acid, of tartaric, malic, fumaric and epoxysuccinic acids by chemical synthesis or microbial transformation. In biotransformations, pure cultures of microorganisms, immobilized cells, enzyme solutions or immobilized enzymes are used. Production of tartaric and malic acids can be accomplished by typical biotransformations with immobilized cells in high yields.