(S)-Hydroxynitrile lyases from Hevea brasiliensis, Manihot esculenta, and Sorghum bicolor and (R)-hydroxynitrile lyase from Prunus amygdalus have been used as biocatalyst for the enantiospecific addition of hydrogen cyanide to aldehydes in organic solvents. The effects of the reaction parameters on the enzymatic reaction rate and product enantiomeric excess (e.e.) are presented. The reaction rate increased with the solvent hydrophobicity but highly hydrophobic solvents were not adapted to high hydrogen cyanide concentrations and provoked loss of activity and product e.e. In the synthesis of 3-phenylpropionaldehyde cyanohydrin catalyzed by (S)-hydroxynitrile lyases from H. brasiliensis, an e.e. value of 88 +/- 1% was obtained under optimized reaction conditions. Lower enantiomeric excess values were obtained under conditions where the enzyme was inactivated: high hydrogen cyanide concentration, high solvent log P, low enzyme loading. At lower temperature (down to -5 degrees C) the e.e. was increased for all four enzymes used. Enzymes from different sources used under identical optimized reaction conditions were found to yield cyanohydrins with very different enantiopurities. This intrinsic enantiospecificity is not an effect of spontaneous reactions independent of the enzyme.