Copolymerizations of ethylene with propene were conducted in toluene employing the soluble catalytic system TiCl4.2THF/AlCl2(C2H5)/Al(i-C4H9)(3) combined with different metalchlorides (CoCl2, ZnCl2 or MgCl2) complexed with tetrahydrofuran (THF). The catalyst activity was strongly dependent upon the metal chloride (MtCl(2)) used as catalyst modifier and its Mt/Ti molar ratio. A clear increase of ethylene/propene copolymerization activity was noted up to Co/Ti and ME/Ti = 2, after which it decreased markedly. The presence of MtCl(2) in the catalyst system had an important effect on the microstructure of copolymers.