Abstract: The invention relates to a method of preparing an optically active cyanohydrin by addition of hydrogen cyanide to a carbonyl compound, selected from aldehydes and ketones, in the presence of hydroxynitrile lyase, in a biphasic solvent system, comprising a homogeneous aqueous solution of said enzyme, and a suitable organic solvent which is at least substantially immiscible with water. Said method is performed in that said homogeneous aqueous solution is buffered with an acetate buffer having a buffer concentration of between 0.005 and 0.1 mole per liter or with a non-acetate buffer, that the volume ratio organic phase:aqueous phase is between approx. 5:1 and approx. 1:5, and that a solution of hydrogen cyanide and said carbonyl compound in said organic solvent is properly mixed during the reaction period with said homogeneous aqueous solution of hydroxynitrile lyase.