A process for preparing enantiomer-enriched heterocyclic (R)- and (S)-cyanohydrins of the formula (I), ##STR00001## where R1, R2, R3, R4 independently of one another are H, an unsubstituted or substituted C.sub.1 C.sub.24-alkyl, alkenyl or alkynyl radical, where one or more carbon atoms in the chain can be replaced by an oxygen atom, a nitrogen atom, a sulfur atom, an SO or an SO.sub.2 group, an unsubstituted or substituted aryl or heteroaryl radical or heterocyclic radical or halogen, hydroxyl, NR5R6, acetyl, oxo, C.sub.1 C.sub.6-carbalkoxy, C.sub.1 C.sub.6-carbalkoxyamino, COOR7, cyano, amide, benzoylamino or NO.sub.2,R5 and R6 are H, C.sub.1 C.sub.6-alkyl radical, phenyl radical, benzyl radical or COOR7, or together form a C.sub.2 C.sub.8-alkylene or heteroalkylene radical,R7 is H or C.sub.1 C.sub.6-alkyl,n is 0, 1, 2 or 3,X, Y and Z can be an unsubstituted or substituted carbon atom or a radical selected from the group consisting of N, O, S or NR5R6, where R5 and R6 are as defined above, SO or SO.sub.2, and at least one of the radicals X, Y and Z is not a carbon atom, where the compounds of the formula (I) can have one or more double bonds in the ring, with the proviso that in a 5-membered ring the double bond is not conjugated with the --C(OH)CN-group, and/or can be anellated and/or bridged,by reacting a ketone of the formula (II), ##STR00002## where R1, R2, R3, R4, X, Y, Z and n have the meanings above, with an (R)- or (S)-hydroxynitrile lyase in an organic, aqueous or two-phase system, or in emulsion, in the presence of a cyanide group donor.