The polymerization of alpha-methylstyrene (AMS) catalyzed by Maghnite-H+ (Mag-H) was investigated. Mag-H is a montmorillonite sheet silicate clay, exchanged with protons. It was found that the cationic polymerization of AMS is initiated by Mag-H at ambient temperature in bulk and in solution. The effect of the amount of Mag-H, the temperature, and the solvent was studied. The polymerization rate increased with increase in the temperature and the proportion of catalyst, and it was larger in nonpolar solvents. These results indicated the cationic nature of the polymerization. It may be suggested that the polymerization is initiated by proton addition to monomer from Mag-H. (C) 2008 Wiley Periodicals, Inc.