The deacylation reactions of p-nitrophenyl propionate and acetate in buffered aqueous solution with and without ammonia and the effects of cetyltrimethylanunnium bromide (CTAB) micelle on them have been investigated. The pseudo first order rate constant of the deacylation reaction in the buffer with ammonia shows higher value than that in the buffer of the same pH but without ammonia. CTAB micelles show positive catalysis on the reaction in the system without ammonia no matter what pH is. But in systems including ammonia, CTAB micelle shows catalytic effect at high pH and negative catalytic effect when the pH is lower than 9.6. These interesting phenomena have been attributed to that the mechanism of deacylation reaction in buffer with and without ammonia are different. Only hydrolysis occurs in the systems without ammonia., whereas both hydrolysis and ammonolysis exist in the systems with ammonia. Actually, CTAB micelle can only enhance the reaction rate of hydrolysis but reduce that of ammonolysis.