A kinetic spectrophotometric investigation of the reaction of the hydrogen peroxide anion with methyl p-nitrophenyl sulfate in methanol solvent resulted in the evaluation of the pKa of HOOH in methanol at 25'C as 15.8 f 0.2. Since normal kinetic procedures for the determination of the equilibrium constant K for the process CH30-+ HzOz i=t CH30H + HOT were found to be associated with high uncertainty, another procedure was devised to establish the magnitude of K. This method is based on an analysis of the changing slopes of plots of pseudo-first-order rate constants against the total base concentration as the stoichiometric amount of hydrogen peroxide is varied. The method is applicable to any system in which anionic nucleophiles generated in situ compete with solvent anions. Such B corroboration of kinetically determined equilibrium constants is believed essential. The kinetic data allow the specific rate constant kHOO-for the reaction of methyl p-nitrophenyl sulfate with hydrogen peroxide anions to be evaluated and yield the rate constant ratio kHOO-/kMeO-= 8.8 f 2.2. This confirms the existence of an (Y effect at saturated carbon in this system.