A process for the production of EPRs comprising contacting ethylene, propylene, and optionally, a diene, under polymerization conditions, with a catalyst system comprising:(A) a catalyst precursor comprising:(i) a vanadium compound, which is the reaction product of(a) VX.sub.3 wherein each X is independently chlorine, bromine, or iodine; and(b) an electron donor, which is a liquid, organic Lewis base in which VX.sub.3 is soluble;(ii) a modifier having the formula BX.sub.3 or AlR.sub.(3-a) X.sub.a wherein each R is independently alkyl having 1 to 14 carbon atoms; each X is as defined above; and a is 1 or 2; and(iii) a support for said vanadium compound and modifier,said catalyst precursor being in an independent or prepolymerized state,wherein said independent catalyst precursor or said prepolymerized catalyst precursor have been pre-oxidized with an oxidant in an amount sufficient to increase the oxidation state of the vanadium by at least one;(B) a cocatalyst consisting essentially of a compound having the formula AlR.sub.(3-a) X.sub.a and, optionally, a compound having the formula AlR.sub.3 wherein R, X, and a are as defined above; and,(C) optionally, a promoter, which is a chlorinated ester having at least 2 chlorine atoms; a saturated or unsaturated aliphatic halocarbon having at least 3 carbon atoms and at least 6 halogen atoms; or a haloalkyl substituted aromatic hydrocarbon wherein the haloalkyl substituent has at least 3 halogen atoms.