The diastereoselectivity for the hydrolysis of p-methoxycarbonylphenyl N-(benzyloxycarbonyl)-D(L)-phenylalanyl-L-leucinate in aqueous solution was inverted by changing concentrations of the substrates or the reaction temperature. The monomer-aggregate transition operated on the LL-substrate, which was suggested by the spectroscopic measurements, seems to be responsible for such behavior.