H2SO4 initiated cationic polymerization of a - methylstyrene was carried out in chloro- jorm. The effect of temperature and monomer concentration on rate of polymerization and on the molecular weight of poly a-methylstyrene were studied. The total activation energy of the reaction was found to be - 32.6 kj mol-1, Also assuming that the transfer reactions were negligeble, it was obtained that kp/ kt was equal to 2.64xl02.