C-13 NMR spectroscopic studies of the in situ polymerization of C-13-enriched ethylene ((C2H4)-C-13) in the presence of Cp2Zr((CH3)-C-13)(2) and methylaluminoxane or B(C6F5)(3) as cocatalysts were carried out. The first direct observation of Cp2Zr-polymeryl, species was made. The in situ polymerization experiments in the presence of different concentrations of dinuclear [(Cp2ZrMe)(2)(mu-Me)](+) and mononuclear cation species [Cp2ZrMe](+), having [MeB(C6F5)(3)](-) or [MeMAO](-) counterions, were performed. These comparisons made it possible to make the assignments of the zirconocene complexes bearing the polymeryl chain as ligands [(Cp2ZrCH2P)-C-13](+)[(13)CH(3)MAO](-), [(Cp2ZrCH2P)-C-13](+)[(CH3B)-C-13(C6F5)(3)](-), and Cp2Zr((CH2P)-C-13)((CH3)-C-13). They are in equilibrium with each other. Mononuclear Zr-polymeryl ion pairs such as [(Cp2ZrCH2P)-C-13](+)[(CH3B)-C-13(C6F5)(3)](-) and [(Cp2ZrCH2P)-C-13](+)[(13)CH(3)MAO](-) either are the propagating active species or are intermediates, closely related to the active species, in the polymer propagation.