Highly filled polyethylene (PE)-based nanocomposites were obtained by in situ polymerization technique. An organically modified montmorillonite, Cloisite (R) 15A (C15A), was previously treated with methylaluminoxane (MAO) to form a supported cocatalyst (C15A/MAO) before being contacted with a zirconocene catalyst. The main features of C15A/MAO intermediates were studied by elemental analysis, TGA, TGA-FTIR, WAXD, and TEM. MAO reacts with the clay, replaces most of the organic surfactant within the clay galleries and destroys the typical crystrallographic order of the nanoclay. The catalytic activity in the presence of C15A/MAO is higher than in ethylene polymerization without any inorganic filler and increases with MAO supportation time. This indicates that part of the polymer chains grows within the clay galleries, separates them, and makes it possible to tune the final morphology of the composites. The polymerization results and the influence of C15A pretreatment and polymerization conditions on thermal and morphological properties of the hybrid PE/C15A nanocomposites are presented. (C) 2008 Wiley Periodicals, Inc.