The present invention relates to a process for preparing optically active cyanohydrins of the formula (II), ##STR1## by reacting an aldehyde of the formula (I) ##STR2## with HCN in a water-immiscible organic solvent in the presence of water, in the absence or presence of a buffer, in the presence of a (R)-hydroxynitrile lyase, where X, Y and Z in formula (II) have the same meaning as in formula (I), independently of each other are identical or different and are H, F, Cl, Br, I, OH, O(C.sub.1 -C.sub.4 -alkyl), OCOCH.sub.3, NHCOCH.sub.3, NO.sub.2 or C.sub.1 -C.sub.4 -alkyl.