Polymerizations of ethylene or propylene were conducted in toluene employing the soluble catalytic system TiCl 4 . THF/AlCl 2 (C 2 H 5 )/Al(i-C 4 H 9 ) 3 combined with different metal chlorides (CoCl 2 ,MgCl 2 or ZnCl 2 ) 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.