Activation of Cp2ZrCl2 and Cp2ZrMe2 by methylaluminoxane (MAO) in toluene is largely complete at Al:Zr ratios of 100:1 to 200:1 as revealed by electrospray ionization mass spectrometry (ESI MS). The anions present undergo chlorination in the case of Cp2ZrCl2. DFT calculations reveal that chlorination of MAO is favorable and involves dissociation of Me3Al, followed by association of Me2AlCl. Ethylene polymerizations were conducted using these catalyst precursors in toluene. The activity vs [Zr] data are essentially identical, while higher MW polyethylene is formed with a narrower MWD at lower Al:Zr ratios in the case of Cp2ZrMe2. The activity vs [Zr] data could be fit to a model which invokes bimolecular deactivation of growing chains. ESI MS reveals that a dinuclear Zr-2 cation with m/z 557 is formed on exposure of [Cp2Zr(mu-Me)(2)AlMe2](+) to ethylene, in addition to other cations that are dinuclear with respect to Zr. Labeling experiments using ethylene-d(4) indicate that these dinuclear cations are derived from ethylene, either through direct incorporation in the case of m/z 557, or indirectly through incorporation of deuterium following e.g. beta-H elimination. These experiments shed light on the need for high Al:Zr ratios for ethylene polymerization using soluble metallocene catalysts. The active catalyst [Cp2ZrR][MAO(Me)] (R = H, Et or a higher homologue) suffers a second order deactivation, and thus activity improves upon dilution of the catalyst precursor at constant [Al].