The activity of metallocene/methylaluminumoxane (MAO) catalysts in olefin polymerization is highly dependent on both the alkylation and activation of the complexes. The leaving ligands have an important role in the complex activation, influencing the activity of the system. The aim of this work was to study the reactions of complexes Me2Si(2-Me-4,5-BenzInd)(2)ZrCl2 (A; BenzInd = benzindenyl) and Me2Si(2-Me-4,5-BenzInd)(2)Zr(Cl)(NEt2) (B) with trimethylaluminum (TMA) and MAO. The reaction kinetics and products were studied by both ultraviolet-visible and NMR spectroscopy. In addition, the polymerization behavior of the different species was investigated in propene polymerizations. Complex B was more easily monomethylated by TMA than complex A and resulted in L2Zr(Me)(NR2)-type species. Monomethylation of the complexes before polymerization enhanced the polymerization activity of both complexes. When complexes A and B reacted with MAO, similar cationic species were formed, giving equal polymerization activities. (c) 2005 Wiley Periodicals, Inc.