Effect of temperature on the cationic polymerization of styrene was investigated by per- forming the polymerization reaction at different temperatures. The total activation energy of the reaction was measured to be 45.14 kj mol-1 between-20°C and + 20°C. The limiting viscosity numbers of ali the polymers prepared at different temperatures were measured. Below room temperature a relationship between the limiting viscosity numbers and the temperature was obtained as follow, log [fl] - 9.80xl0~2 -i 2.70 The polymers were prepared in four different solvents and the rate of polymerization in each solvent was measured. Between the rates of polymerization and the dielectric constants of the solvents, the following equations were obtained. , / d [M] \ log I -r-M = —4.42 + 0.123 D \ dt / , / d [M] \ D—1 log ( i—| = —5.11 + 4.208 & \ dt y 2 D+l Polymerization reactions were carried out in air and nitrogen atmosphere. The rates of polymerization and the average molecular weights of the polymers were found the same in these experiments.