Abstract: A process for preparing enantiomerically pure alcohols of the formula I (Ia or Ib) where the substituents have the following meanings: R1 hydrogen or substituted or unsubstituted C1-C6-alkyl, C1-C6-alkoxy or C1-C6-alkanoyl, R2 and R3 independently of one another hydrogen or substituted or unsubstituted C1-C6-alkyl, C1-C6-alkoxy, C1-C6-alkanoyl, C1-C6-alkylthio, C1-C6-alkylsulfinyl or C1-C6-alkylsulfonyl, R4 substituted or unsubstituted C1-C6-alkyl, C3-C8-cycloalkyl, which comprises reducing compounds of the formula II where the substituents R1 to R4 have the abovementioned meanings  in aqueous solution in the presence of a carbon source and of a microorganism or of a reducing agent, of a cofactor and of an enzyme, to compounds of the formula I.