A process for producing EP and EPDM polymers with a reduction in fouling is provided, which process comprises polyemrizing ethylene, an alpha-olefin and a nonconjugated diolefin (if utilized) in a polymerization medium comprising a hydrocarbon diluent at a temperature of from about -50.degree. C. to about 150.degree. C. in the presence of a catalyst system comprising (a) a vanadium compound, (b) diethyl aluminum chloride, ethyl aluminum sesquichloride or ethyl aluminum dichloride and (c) an organo aluminum compound of the formula R.sub.3 Al wherein R is an alkyl group having from 8 to 12 carbon atoms and, optionally, triethyl aluminum, the molar ratio of chloride in diethyl aluminum chloride to the total aluminum content in (b) and (c) being from about 0.7:1 to about 0.95:1, the molar ratio of the chloride in ethyl aluminum sesquichloride to the total aluminum content in (b) and (c) being from about 0.7:1 to about 1.4:1 and the molar ratio of the chloride in ethyl aluminum dichloride to the total aluminum content in (b) and (c) being from about 0.7:1 to about 1.8:1 and the ratio of the mols of vanadium compound to the total mols of diethyl aluminum chloride and ethyl aluminum sesquichloride being from about 1:10 to about 1:50 to the desired conversion.