The activity of metallocene catalysts in ethylene polymerization can be increased by a factor of up to 3.5 when the corresponding metallocene catalyst precursors possess one or two bulky alkyl ligands such as benzyl groups instead of the conventional chloro ligands. We assume that in the activation process with methylalumoxane (MAO) these bulky alkyl groups are transferred from the metal to the MAO cocatalyst. The actual cocatalyst is supposed to be a cage with one or more monomeric AlMe3 molecules inside. The bulky alkyl group is transferred to such an AlMe3 unit and can act as a spacer separating the catalytically active metallocenium cation from the MAO anion. This effect is supposed to be responsible for the increase in activity. (C) 2003 Wiley Periodicals, Inc.