In recent years, there has been increasing theoreti cal and practical interest in mixed surfactant based micellar systems [1][2][3][4][5]. This is primarily due to the uncommon physicochemical and mechanical and other properties shown by these mixed compositions. Furthermore, these compositions are cheaper and less sensitive to the purity of their components than indi vidual solutions. The mixed micellar systems are of both fundamental and practical interest, The mixed solutions differ in their properties (critical micelle concentration (CMC), aggregation number, solubiliz ing capacity) from the individual solutions of their constituent surfactants [2, 6-9]. This makes it possible to attain the required parameters of the micellar sys tem by varying its composition. In most cases, mixed micellar systems are superior to individual compounds since the composition of the mixture can be optimized for each particular case. Although the number of pub lications on mixed micellization, including studies using theoretical and semiempirical approaches, has increased in the last 2-3 years, the basic features of the functioning of binary surfactant systems still need a more profound consideration. The reactivity of com pounds in mixed systems remains a poorly explored area [10][11][12][13][14][15][16][17].Earlier, we studied the micellization properties and catalytic effects of binary micellar systems based on the cationic surfactant cetyltrimethylammonium bro mide (CTAB). The following nonionic surfactants were examined as the second component: polyethyl ene glycol(~9) monotetradecyl ether [18], polyethyl ene glycol(10) oleyl ether (Brij 97) (C 14 E 9 ) [19], and polyethylene glycol(10) mono 4 isooctylphenyl ether [20,21]. These surfactants differ in their hydrophiliclipophylic balance (HLB). The present work continuous this series of studies, dealing with the CTAB-Brij 35 binary system. Its main purpose is to estimate the cat alytic activity of the system in the alkaline hydrolysis of O ethyl O p nitrophenyl chloromethylphospho nate (1), Abstract-The micellization and surface properties, Stern layer micropolarity, and catalytic action of the cetyltrimethylammonium bromide-Brij 35 system in the alkaline hydrolysis of O ethyl O p nitrophenyl chloromethylphosphonate have been investigated at different proportions of the constituent surfactants. This system has unique, invertible catalytic properties ranging from acceleration of the hydrolysis reaction by more than one order of magnitude to strong inhibition of this reaction and even termination of the reaction at certain proportions of the surfactants. The factors in the changes of the catalytic effect under variation of the surfactant ratio are micropolarity in the reaction zone and surface potential. A likely cause of the termi nation of the process is the shift of the reaction zone from the surface layer to the micelle core, where the envi ronment is less polar.