The polymerization of methyl methacrylate in aqueous montmorillonite suspension with thiourea was studied. The polymerization initiated on the montmorillonite surface following the adsorption of thiourea and the monomer. A 20% increase in the basal spacings of the clay mineral was observed. A large fraction of the polymer was nonextractable and was found to be thermally more stable than the extractables. The rate of the polymerization increased with increasing amounts of clay, thiourea, and followed one-half order on the latter. The apparent energy of activation (Ea) of the system was characteristic of redox free radical polymerization.