The addn. of a miscible org. solvent, which decreases the water concn. of the system, is considered as a means to reduce hydrolysis when transesterification is the desired reaction. This approach is illustrated by the study of a model reaction, the kinetically controlled transesterification of a racemic mixt. of phenylalanine Pr ester with 1,4-butanediol by chymotrypsin (EC 3.4.21.1) in the presence of the miscible org. solvent, DMSO. The results showed that no appreciable effect on the transesterification productivity or on the initial differential and integral selectivity is obsd. for DMSO concns. up to 20%. At higher concns., the transesterification productivity diminishes while the initial differential and the integral selectivity remain essentially unchanged. These observations can be explained by attributing an inhibitory role to DMSO acting on the formation of the acyl-enzyme intermediate. [on SciFinder (R)]