Critical micelle concentrations were measured for mixed micelles of sodium decyl phosphate (NaDeP) and sulfobetaines (CnH2n+1N+Me2(CH2)(3)SO3-, SB3-n, n = 10, 12, 14, 16), and values of pH(app) were calculated from spectroscopic data on the extent of deprotonation of 1-dodecylpyridinium-3-aldoxime bromide. For 0.1 M total surfactant, rate constants of hydrolyses of 2,4-dinitrophenyl acetate and octanoate (DNPA and DNPO, respectively) and benzoic anhydride (Bz(2)O) were fitted quantitatively in terms of reactions of OH- and decyl phosphate ion on the assumption that sulfobetaine micelles increased the reactivity of decyl phosphate ion.