Three vanadocene-cocatalyst catalytic systems for ethylene polymerization-Cp2ZrCl2/Al2Et3Cl3 [dichlorobis(eta -cyclopentadienyl)vanadium/ethylaluminumsesquichloride], Cp(2)ZrCl(2)MAO [dichlorobis(eta -cyclopentadienyl)vanadium/methylaluminoxane] and Cp2ZrCl2AlEt3 [dichlorobis(eta -cyclopentadienyl)vanadium/triethylaluminum]-were monitored by electron spin resonance (ESR) spectroscopy. It was found that at least a certain kind of vanadium complex is formed after mixing dichlorobis(eta -cyclopentadienyl)vanadium with ethylaluminumsesquichloride. After introducing ethylene, a new kind of vanadium complex is detected by ESR. Ethylene can be polymerized by using a dichlorobis(eta -cyclopentadienyl)vanadium/ethylaluminumsesquichloride catalytic system. These results possibly indicate that the vanadium complex exists in the forms ClCp2VClEtAlCl2 and ClCp2VClEtAlClEt, which are responsible for forming active centers. The Dichlorobis(eta -cyclopentadienyl)vanadium and dichlorobis (eta -cyclopentadienyl)vanadium/triethylaluminum systems also were monitored by ESR. Completely different spectra were recorded compared to those of the dichlorobis(eta -cyclopentadienyl) vanadium/ethylaluminumsesquichloride system. Ethylene almost cannot he polymerized by the dichlorobis(eta -cyclopentadienyl)vanadium/methylaluminoxane and dichlorobis(eta -cyclopentadienyl)vanadium/triethylaluminum catalytic systems, showing that resultant vanadium complexes of dichlorobis(eta -cydopentadienyl)vanadium/methylaluminoxane and dichlorobis(eta -cyclopentadienyl)vanadium/triethylaluminum catalytic systems are different from that arising from the dichlorobis(eta -cyclopentadienyl)vanadium/ethylaluminesquichloride system. Plausible mechanisms are suggested for this.