A concentrated microaqueous solution of the amino acids N-(benzyloxycarbonyl)-l-aspartic acid (ZA) and l-phenylalanine methyl ester (PM) was used at a suitable pH for the enzymic synthesis of ZAPM. Dehydrochlorination of PM?HCl was first carried out by aqueous/chloroform biphasic extraction, then the chloroform phase which extracted PM was concentrated with a rotary evaporator. In this operation, concentrated dehydrochlorinated PM solution at a concentration higher than 3.5 M could be prepared. The pH value of the reaction media was regulated by setting the ratio of PM/ZA. High enzymic activity of ZAPM synthesis was observed at a PM/ZA ratio of 3.3 which gives the suitable pH value for the enzyme. Water addition of 100 μl in the reaction media consisted of 1.0 ml of PM solution (3.77 M) and 300 mg of ZA (1.13 mmol) was effective in increasing enzymic activity.