Slurry polymerizations of ethylene over vanadium catalysts (based on VCl4 and VOCl3) and their MgCl2(THF)(2)-supported equivalents were studied. Unsupported vanadium catalysts were found to be unstable while the vanadium active sites deposited on the MgCl2(THF)(2) complex are stable. A sharply outlined correlation was found between the concentration of vanadium(LU) and catalyst productivity. The high activity and stability of the vanadium catalyst when supported on the magnesium complex is attributed to the increase of resistance to reduction of active vanadium(III) to inactive vanadium(II) by an organoaluminium co-catalyst.