The interaction of strong acids with hydroperoxides at room temperature has been shown to result in the formation of radical species capable of initiating vinyl polymerization. For the reaction of tert-butyl hydroperoxide (tBHP) with sulfuric acid in methyl methacrylate (MMA), the rate of initiation of polymer varies in a complex manner with the concentration of the components but is first-order in [tBHP] and approximately second-order in [H2SO4] at low concentrations of tBHP and in the presence of excess acid. At high concentrations of tBHP, chain transfer to unprotonated hydroperoxide retards the rate of polymerization. The rate of polymer initiation parallels the ionization ratio [BH+]/[B] of a weak base (B = 4-chloro-2-nitroaniline) in MMA-H2SO4and the high order in [H2SO4] can be explained in terms of the requirement of more than one molecule of H2SO4for the monoprotonation of the hydroperoxide (homoconjugation). Endgroup analysis of the polymers has shown that the initiating species are the tert-butoxy and hydroxy radicals. These data are consistent with the homo-lytic scission of the O-O bond of the hydroperoxide being facilitated by protonation of one of the oxygen atoms. Comparison of the rate of polymer initiation with the overall rate of decomposition of tBHP has indicated that as much as 30% of the hydroperoxide decomposes by a radical path.