The aggregation behavior and catalytic activity toward nucleophilic substitution in p-nitrophenyl acetate of the polyethyleneimine (PEI) + cationic surfactant systems is studied. The values of the critical aggregation concentrations are shown to be much lower as compared to critical micelle concentrations of single solutions. A decrease in the self-diffusion coefficient of surface-active ions is observed within the concentration range limited by the tensiometry critical points. The data obtained provide evidence for mixed aggregation in the PEI + cationic surfactant systems. Due to the contribution of specific interactions, surfactants with hydroxyethylated head groups cetyldimethyl(2-hydroxyethyl)ammonium bromide and cetylmethylbis(2-hydroxyethyl)ammonium bromide demonstrate a higher affinity toward PEI as compared to cetyltrimethylammonium bromide. The systems studied show catalytic activity toward the cleavage of p-nitrophenyl acetate, which increasees with the number of hydroxyethyl fragments in their head groups and reaches more than 2 orders of magnitude.