A process model for the enzymatic synthesis of (R)-cyanohydrins in an aqueous-organic biphasic-stirred tank reactor was developed. The conversion of benzaldehyde into (R)-mandelonitrile, catalyzed by Prunus amygdalus hydroxynitrile lyase at 5 degreesC and pH 5.5, was chosen as a model system with methyl tent-butyl ether as the organic phase. The process model consisted of a description of the reaction kinetics, mass transfer kinetics, and the mass balances for both the aqueous and the organic phase. Values for the enzyme kinetic parameters, according to ordered bi-uni kinetics, the lumped mass transfer coefficient for benzaldehyde, and the partition coefficients were determined separately. The process model is validated by using 11 experimental data sets of batch conversions in the aqueous-organic biphasic-stirred tank reactor. In these 11 experiments, different enzyme concentrations and phase volume ratios were used. The model was found to be valid with respect to both the conversion and the enantiomeric excess. To synthesize cyanohydrins with a high enantiomeric excess, the enzyme is required to work at mass transfer limited conditions. The developed process model will be used to investigate other process concepts and other substrates.