In this paper, an electrochemical study of the metallocene/methylaluminoxane (MAO) and metallocene/trimethylaluminium (TMA) species is presented. The metallocenes used were [(CH3)(2)Si(Cp)(2)ZrCl2] and [(Cp)(2)ZrCl2]. The evaluation of the Metallocene/MAO and Metallocene/TMA systems was based on the analysis of the redox potentials of the solutions with different Al/Zr molar ratios using acetonitrile as solvent. The results obtained for Al/Zr ratios lower than 30, suggested that in presence of TMA or MAO, the formed monomethylated species are [(L)Zr-IV(CH3)Cl], L = cyclopentadienyl (Cp)(2) or (CH3)(2)Si(Cp)(2). For Al/Zr molar ratios higher than 30, the results obtained in TMA presence are in agreement with the formation of the species [(L)Zr-IV(CH3)(mu-CH3)Al(CH3)(2)](+), while in MAO presence the ionic pair, [(L)(ZrCH3)-C-IV](+)[MAOCl](-), is formed. There's a general agreement that these are active species in olefin polymerization. (C) 2003 Elsevier B.V. All rights reserved.