Abstract: A solution process, and a catalyst therefor, for the preparation of high molecular weight polymers of alpha-olefins viz. homopolymers of ethylene and copolymers of ethylene and C.sub.3 -C.sub.12 alpha-olefins, is disclosed. The process is operated under solution polymerization conditions at 105.degree.-320.degree. C. The catalyst is obtained from titanium tetrahalides, vanadium oxytrihalides and organoaluminum compounds, in which an admixture of catalyst components is heat-treated at 180.degree.-250.degree. C. and subsequently cooled to a temperature of less than 150.degree. C. Additional vanadium oxytrihalide is then added. The resultant catalyst solution is activated with an aluminum compound. The catalyst has good activity, showing superior activity to catalysts in which additional vanadium oxytrihalide is not added or added at temperatures above 150.degree. C.