It is noted that the diastereoselective specificity for the hydrolysis of dipeptide esters having phenylalanine (Phe) or leucine (Leu) residues in aqueous solution systems should correcpond to the concentration of substrates and reaction temperature. In particular, the markedly high diastereoselectivity is observed for the hydrolysis of p-methoxycarbonylphenyl N- (benzyloxycarbonyl) -D(L) phenylalanyl-L-phenylalaninate (Z-D(L) -Phe-L-Phe-PMCP), and the diastereoselectivity is maximized by regulating the concentration of substrates and reaction temperature. On the other hand, such a diastereospecificity is not observed for the hydrolysis of p-methoxycarbonylphenyl N-(benzyloxycarbonyl) -D (L) -leucyl-L-leucinate (Z-D(L) -Leu-L-Leu-PMCP). The spectroscopic study suggests that the high diastereoselectivity for the spontaneous hydrolysis of Z-D(L) -Phe-L-Phe-PMCP is originated mainly from the monomer-aggregates transition of diastereomeric esters in aqueous solution systems.