The kinetics of the aminolysis of selected phenyl acetates in an excess of poly(ethylenimine) (PEI) were studied at 25 degrees C in aqueous solution at various pH values. The neutral esters a-nitrophenyl acetate (NPA) and 2,4-dinitrophenyl acetate (DNPA), showed saturation kinetics vs PEI concentration. The substrate-polyelectrolyte binding constant (K-1) as well as the decomposition rate constant (k(cat)) of the complex (X-1) which yields acetylated PEI and phenol were determined. The aminolysis of 4-acetoxy-3-nitrobenzenesulfonate (ANBS) and 3-acetoxy-2,6-dinitrobenzoic acid (ADNB) showed a more complicated rate behavior: the rate increased as the PEI concentration increased, passed through a maximum, and then decreased. These results were explained by the formation of the ordinary complex (X-1) and by an additional unreactive complex (X-2). An equation was elaborated that allowed the evaluation of (a) the binding constant (K-1), (b) the decomposition rate (k(cat)) of X-1, and (c) the unreactive complex inhibition constant (K-2). Unlike the case of neutral esters, the Bronsted-type plots obtained for the ANBS and ADNB complex decomposition (X-1) were not linear. The most likely mechanism involves the formation of a zwitterionic tetrahedral intermediate T+/- (Scheme 3) along the reaction path. A semiempirical equation based on this hypothesis fits the experimental data well. A detailed examination of a possible more general reaction scheme, the estimation of pK values of intermedates and structure-reactivity considerations, showed that intramolecular general base and/or general acid catalysis are unimportant. Solvent effect and activation parameters were discussed.