Polymerization reactions of styrene in toluene at 60 degreesC have been carried out using Ziegler-Natta catalysts containing praseodymium trichloroacetate-triethyl aluminum [Pr(OCOCCl3)(3)-Et3Al] (system I) and praseodymium trichloroacetate-diethyl aluminum bromide [Pr(OCOCCl3)(3)-Et2AlBr] (system II). The reaction was of first order with respect to both monomer and catalyst concentrations. At a stoichiometric ratio of Al/Pr = 8, an optimum catalytic activity was observed. The activation energies for systems I and II ranged between 21.06 kJ/mol and 13.79 kJ/mol, respectively. System I was white in color and system II is waxy. Molar masses of systems I and II were 9.1 x 10(5) and 2.2 x 10(4), respectively. The polymers were characterized by viscometry and H-1 NMR methods.