The effect of hydrogen on the molecular weight of polyethylene obtained over vanadium catalysts (based on VCl4, and VOCl3) supported on MgCl2,(THF)(2) was studied and the results were compared to those obtained for similar titanium catalysts. It was confirmed that the dependencies of the transfer reaction on the hydrogen concentration are a half-order in all investigated systems. However, the transition metal of the catalytic site affects the ratio of the transfer rate with hydrogen to the propagation rate (k(tr,H)/k(p)) and the results showed that hydrogen is a more effective agent of polyethylene molecular weight control in vanadium-based systems as compared to the titanium catalyst.