In order to develop a new reaction-control system with artificial enzymes, we investigated the hydrolytic cleavage of p-nitrophenyl esters (CnPNP) catalyzed by hydroxamic acids (HA) in cationic micellar (CTAB) and reversed micellar (CTAB) aggregate systems in aqueous solutions and organic media, respectively. It is noted that the remarkably highly rate-enhancement for the long-chained substrate (p-nitrophenyl hexanoate, C6PNP) with the long-chained catalyst (2-hexyldecanohydroxamic acid, HDHA) in the CATB micellar systems should be fairly related to a favorable orientation of reactants through the hydrophobic interaction between C6PNP and comicelles of CTAB and HDHA. On the other hand, it is found that the short-chained substrate (p-nitrophenyl acetate, C2PNP) is most effective for its hydrolytic cleavage with the hydrophilic catalyst (benzohydroxamic acid, BHA) in the CTAB reversed micellar system. This might be attributable to a higher distribution ratio of the C2PNP into the inner aqueous phase than those of longer-chained substrates.