The stereospecific hydrolysis of phenylalanine esters by chymotrypsin can avantageously be undertaken in aq.-org. 2-phase systems. The partitioning of the deprotonated form of the substrate changes the pH characteristics of the reaction system drastically. It even permits ester hydrolysis to be performed without pH control. Thus, plug flow reactors may be used in the case of immobilized chymotrypsin. However, severe mass transfer limitations are encountered when a fixed bed reactor is fed with an emulsion. [on SciFinder (R)]