An enzymic transesterification was carried out in a continuously operated fixed bed reactor. The reaction system consisted of immobilized a-chymotrypsin catalyzing the transfer of the L-phenylalanine radical from the racemic Pr ester to 1,4-butanediol, yielding L-phenylalanine 4-hydroxybutyl ester. The desired reaction was accompanied by alcoholysis due to the presence of PrOH liberated during the reaction and by hydrolysis of both the Pr and the hydroxybutyl ester. The problem of shifting pH during the reaction due to ester hydrolysis was overcome by adjusting the initial pH of the substrate feed soln. appropriately in order to obtain a sufficiently high buffer capacity provided by the free amino group of the esters. Thus, it was possible to work with shifting pH, and obvious disadvantage for operating reactors of low backmixing for this kind of reaction system. The overall reaction scheme was characterized by the appearance of a max. ester yield as a function of the operating time in case of batch reactors. Surprisingly, the yield became const. as a function of space-time for continuous operation due to a steeper pH drop. The max. productivity achieved with respect to the hydroxybutyl ester was .apprx.65 mol/L-day. [on SciFinder (R)]