The copolymerization of ethylene with highly active TiCl4/MgCl2-supported catalysts in solution reactors at 185°C and 400 Psig pressure is presented. The performance of these highly active supported catalysts at high reaction temperature is characterized by a high initial rate that decays rapidly within the 10 min polymerization time period. In the presence of hydrogen and a comonomer, catalyst yields up to about 300 kg/g (Ti) are achieved. Kinetic data obtained on the influence of a comonomer, e.g., 1-octene or 1-hexene, indicate rate enhancement when used in moderate concentrations. Higher concentrations of comonomer result in a decreasing rate of ethylene consumption. Comonomer/ethylene molar ratios in the range 0–0.827 resulted in comonomer incorporation up to about 2.6 mol % and a small reduction in the polymer molecular weight. © 1993 John Wiley & Sons, Inc.