Polymerizations of ethylene or propylene were conducted in toluene employing the soluble catalytic system TiCl4.THF/AlCl2(C2H5)/Al(i-C4H9)(3) combined with different metal chlorides (CoCl2,MgCl2 or ZnCl2) complexed with tetrahydrofuran (THF). The catalyst activity was strongly dependent upon the metal chloride used as modifier. A clear increase of catalyst activity in ethylene polymerization was observed when the Co/Ti or Mg/Ti mole ratio was increased up to 2 and then decreased markedly. In a similar way, the catalyst activity in propylene polymerization was strongly dependent upon the amount and kind of metal chloride used as modifier.