The reaction temperature was optimized for the enzymatic synthesis of (R)-mandelonitrile in a biphasic system of aqueous buffer and methyl tert-butyl ether by including temperature effects in an existing process model. (R)-Mandelonitrile was synthesized by addition of hydrogen cyanide to benzaldehyde using Prunus amygdalus hydroxynitrile lyase as the catalyst. The reaction was investigated within a temperature range of 5 to 25degreesC and at pH 5.5. At low temperatures the nonenzymatic reaction is reduced to a larger extent than the enzymatic reaction. The mass transfer rate is reduced to a smaller extent. Mass transfer limitation is required for a high enantiomeric excess and determines the conversion rate. Therefore the volumetric productivity decreases at lower temperatures. The equilibrium constant is considerably higher at low temperatures resulting in a higher extent of conversion, or a lower hydrogen cyanide requirement. Both the volumetric productivity and the required enzyme concentration increase by increasing the reaction temperature and aqueous-phase volume while meeting the required conversion degree and enantiomeric excess. For the conversion of 750 moles benzaldehyde per m(3) into (R)-mandelonitrile with an enantiomeric excess of 99.00% and an 98.00% extent of conversion in benzaldehyde, an economic optimum was calculated. A reaction temperature of 15degreesC and an aqueous-phase volume of 50% containing 1.5 kg enzyme could lead to a volumetric productivity of 130 kg (R)-mandelonitrile m(-3) (.) h(-1).