Abstract: An enzymatic reaction of aliphatic, aromatic or heteroaromatic aldehydes or ketones with hydrocyanic acid to form the corresponding (R)-cyanohydrins or (S)-cyanohydrins is carried out in an aqueous medium in the presence of (R)-oxynitrilase (4.1.2.10) or oxynitrilase (4.1.2.11) under acid conditions and at temperatures such that the competing chemical reaction and racemization are negligible compared with the enzymatic synthesis. The process is preferably carried out at a pH of .ltoreq.4.5 and at reaction temperatures between -5.degree. C. and +50.degree. C. The reaction is preferably carried out continuously, advantageously in an enzyme membrane reactor. Of particular interest is the conversion of aromatic aldehydes into the corresponding optically active cyanohydrins.