The behavior of two differently prepared methylaluminoxanes (MAOs) on the activation process of racEt(Ind)(2)ZrCl2 for olefin polymerization was studied. Contrarily to commercial product, the two MAOs used are characterized by a low content of free trimethylaluminium (TMA). MAO-A was prepared by pumping off the TMA contained in commercial MAO, whereas MAO-B was obtained by thermal treatment of TMA with benzophenone. The different elementary steps in the zirconocene activation process depending on the amount and nature of added MAO (commercial and modified ones) were investigated by UV/visible spectroscopy. In all cases, monomethylation of racEt(Ind)(2)ZrCl2 rapidly takes place after addition of small amounts of MAO. Then, abstraction of the second chloride of the metallocene occurs upon addition of larger amounts of MAO (Al/Zr < 200) leading to the formation of highly active catalytic species for hexene polymerization. This is characterized by a bathochromic shift of the zirconocene absorption band. The role of TMA is clearly pointed out by comparing the modified MAO/zirconocene systems with the one based on commercial MAO. The influence of structure and composition of MAO on the activation process is also clearly underlined by the very different amounts of activator needed to get metallocene active species in the presence of commercial and modified cocatalysts. (C) 2002 Elsevier Science B.V. All rights reserved.