In this study a -methyi styrene has been polymerized using sulfuric acid as catalyst. The effects of catalyst and cocatalyst (water) concentration, type of solvent were investigated. It has been found that the rate of the polymerization increases linearly with the catalyst concentration. On the other hand, molecular weight of the polymer was not effected by the catalyst concentration. a -methyi styrene has also been polymerized using different amounts of water as cocatalyst. Our resuîts shöwed that as the water concentration was increased the rate of polymeriza¬tion and the molecular weight of the polymer decreased. a -Methyi styrene has been polymerized at two diffecent temperatures by using ethylene chloride and methylene chloride as solvent. Rate of polymerization and molecular weight of the polymers prepared in ethylene chloride were found greater then that prepared in methylene chloride.