We have determined the second order rate constants for the iodosobenzoate catalyzed hydrolysis of p-nitrophenyl diphenyl phosphate in microemulsions of n-hexadecane, toluene or n-hexylbenzene and water stabilized by cetyl trimethyl ammonium chloride or bromide and different cosurfactants including n-butanol, N,N-dibutylformamide, Adogen 464 and several pyrrolidinones. Our results suggest that the predominant effect of these media on the hydrolysis reaction is electrostatic. Hence, we have extracted estimates of the surface potentials of the microemulsion droplets from rate constants.