Enzymatic process for the enantioselective preparation of optically active cyanohydrins

An enantioselective process for the preparation of the S-enantiomer of an optically active cyanohydrin which comprises reacting an aldehyde or an asymmetric ketone with a cyanohydrin of the formula R.sub.1 R.sub.2 C(OH) (CN) in which R.sub.1 and R.sub.2 independently of one another denote hydrogen or a hydrocarbon group which is unsubstituted or substituted with groups which are inert under the reactions conditions or R.sub.1 and R.sub.2 together denote an alkylene group having 4 or 5 C-atoms, where R.sub.1 and R.sub.2 are not simultaneously hydrogen, in a diluent in the presence of an S-hydroxynitrile lyase of the genus Hevea or Sorghum and isolating cyanohydrin formed, from the reaction mixture.