A process for the production of an ethylene homopolymer or copolymer comprising passing ethylene with or without at least one alpha-olefin comonomer having 3 or more carbon atoms into contact with a mixed catalyst system, under polymerization conditions, said mixed catalyst system comprising:(a) the reaction product of (i) a vanadium halide having the formula VX.sub.3 wherein X is chlorine, bromine, or iodine and each X is alike or different; (ii) a modifier having the formula BX.sub.3 or AlR.sub.(3-a) X.sub.a and (iii) an electron donor(b) one of the following:(i) a complex having the formula ZrMg.sub.b X.sub.c (ED).sub.d ; or(ii) a vanadium oxy compound having the formula VOX.sub.3, VOX.sub.2, VOX, or VO.sub.2 X wherein X is as defined above, or VO(OR).sub.3(c) a hydrocarbyl aluminum cocatalyst; and(d) a halocarbon promoter having the formula R.sub.e CX.sub.(4-e) wherein R is hydrogen or an unsubstituted or halo substituted alkyl radical having 1 to 6 carbon atoms; each R is alike or different; X is chlorine, bromine, iodine, or fluorine; each X is alike or different and e is 0, 1, or 2 provided that, if no fluorine is present, e is 2.