The activation process by methylaluminoxane (MAO) of various zirconocene precursors of the type racEt(Ind)(2)ZrX2, With different extractable X ligands (X = Cl, Me, NMe2, CH2Ph), was investigated both by means of UV-VIS spectroscopy and through hex-l-ene polymerization kinetics. The different elementary steps yielding olefin polymerization active species, i.e. methylation and cationization of the transition metal could be readily discriminated. Unlike racEt(Ind)(2)ZrCl2 precursor for which high amount of MAO (Al/Zr = 2000) is required to form catalytic species active towards olefin polymerization, the complete activation of other racEt(Ind)(2)ZrX2 tested (with X = Me, NMe2, CH2Ph) is readily achieved at relatively low Al/Zr ratios (Al/Zr = 40-50). The corresponding zirconocenium species exhibit different catalytic activity according to the nature of the extracted X ligand, showing the important role of the formed [MAO-X](-) counter-anion in the activation process and more particularly, its ability to still coordinate with the metal center and compete with the incoming olefin.