Enzyme catalysed hydrolyses of dialkylated propanedioic acid diesters have been studied. A novel change of enantioselectivity from pro-S to pro-R in the hydrolysis of ester groups was observed, depending on the chain length of the alkyl substituents of the substrate. Optically pure (S)-(-)-α-methylphenylalanine was prepared by α-chymotrypsin catalysedhydrolysis of benzylmethylpropanedioic acid dimethyl