The leaching of the catalyst zirconocene dichloride (Cp2ZrCl2) from an SBA-15 silica support during ethylene polymerization was studied; severe leaching was observed when commercial methylaluminoxane (MAO) was used as the cocatalyst. However, the removal of free trimethylaluminum (TMA) from an MAO solution with a sterically hindered phenol reduced the catalyst leaching by 97-100%. The results obtained from the leaching experiments with TMA-free MAO Suggested that the major reason for catalyst leaching was the free TMA in the commercial MAO solution, not the pure MAO itself. (c) 2006 Wiley Periodicals, Inc.