Catalysts for the polymerization of ethylene, which contain rac-Me2Si(Ind)(2)ZrCl2 (SBIZrCl2) bound to methylalumoxane-pretreated SiO2 supports (SiO2/MAO/SBIZrCl2). were studied with respect to leaching of zirconocene and MAO into heptane solutions caused by triisobutylaluminum (TIBA). The latter is found to solubilize surface-bound MAO and zirconocene components, such that the Al content in the solid catalysts is reduced by 15-40 wt% and their Zr content by 50-60 wt%. Consequently, ethylene polymerization with such TIBA-containing catalyst systems occurs both on the surface of the solid catalyst and in the solution phase. Polymerization in solution is favored by high MAO contents in the SiO2 carrier. Polymerization in solution leads to polyethylene (PE) products with lower molecular masses than polymerization on the support surface: this is the main cause of formation of PE with bimodal molecular mass distribution when ethylene is polymerized with SiO2/MAO/SBIZrCl2 catalysts co-activated by TIBA. It is proposed that the co-activator TIBA interacts with surface-bound MAO to modify MAO in such a manner that it becomes soluble in aliphatic hydrocarbons and capable of solubilizing some active zirconocene component. (C) 2011 Elsevier B.V. All rights reserved.