Modified dipeptides of the type AcXYNH2 were synthesized by alpha-chymotrypsin in reversed micelles of tetradecyltrimethylammonium bromide in heptane/octanol using substrates AcXOEt (X = Phe, Tyr, Trp) as acyl donors and YNH2 (Y = Gly, Ala, Ser, Val, Leu, Ile, Phe) as acyl acceptors. With the exception of TyrPhe, all syntheses were accomplished with yields ranging from 24-96% in less than 1 h. The dipeptides PheLeu, PheIle, PheVal, TyrIle, and TyrVal precipitated in the media during reaction. The transport of the ester substrates from the organic bulk to the water pool of the micelles was identified as the rate-limiting step. The amide nucleophilicity was found to decrease with an increase in hydrophobicity : Ser > Ala > Gly > Val > Ile > Leu. A correlation between the dipeptide yields and their hydrophobicity, accessible surface area, and molecular weight showed that smaller and more hydrophilic dipeptides were synthesized with higher yields.