UV/vis in diffusion reflection mode (DRS) and DRIFT spectroscopy have been used to study the surface zirconocene species formed at the interaction of Me2Si(Ind)(2)ZrCl2 and MC2Si(Ind)(2)ZrMe2 complexes with the MAO/SiO2, support. Effect of additional activation of these catalysts with TIBA has been studied as well. Structure of type [Me2Si(Ind)(2)ZrMe](+)[MeMAO](-) (C) is formed at the reaction of Me2Si(Ind)(2)ZrMe2 complex with MAO/SiO2 (a.b. at 456 nm in UV/vis spectra). Interaction of this complex with TIBA results in the formation of new structure (D) with a.b. at 496 nm in UV/vis spectra. The surface species of different composition and structures are formed at interaction Of Me2Si(Ind)(2)ZrCl2 complex with MAO/SiO2. The ratio between these species depends on the zirconium content in the Me2Si(Ind)(2)ZrCl2/MAO/SiO2 catalysts. According to the DRIFTS data (CO and ethylene adsorption) and ethylene polymerization data these catalysts contain active Zr-Me bonds but activity of these catalysts at ethylene polymerization is low. Interaction of Me2Si(Ind)(2)ZrCl2/MAO/SiO2 with TIBA leads to the formation of the new cationic structure of type (D) with a.b. at 496 nm in UV/vis spectra and great increasing of activity at ethylene polymerization. (c) 2005 Elsevier B.V. All rights reserved.