The polymerization of methyl methacrylate electroinitiated at the cathode was examined in concentrated solutions of the monomer containing aqueous sulfuric acid. The initiating reaction is not due to the formation of hydrogen atoms, as demonstrated by varying the nature of the cathode and of the acid, but involves the participation of the polymeric peroxide contained in the monomer. The cathodic material greatly affects the molecular weights and the yields, as does the composition of the solutions. Lead cathodes, which proved to be particularly efficient, showed phenomena of adsorption of both monomer and polymer. Kinetic curves are similar to those obtained in conventional radical polymerizations of MMA; a marked gel effect, limiting the termination reaction and leading to very high molecular weights, was observed.