The organic aluminum compounds (OACs) have been prepared on the surface of clay such as montmorillonite K-10 (MMT-10) and Kunipia by reaction of partial hydrolysis of trimethylaluminum (TMA). The effect of the water and acidity of the clay in ethylene polymerization was investigated over Cp2ZrCl2 supported on the TMA-modified clay. Irrespective of whether water was present or not in the clay, Cp2ZrCl2 supported on the TMA-modified clay showed catalytic activity for ethylene polymerization when acidic MMT-10 was used. However, in the case of basic Kunipia, no catalytic activity for ethylene polymerization was obtained. When Kunipia was changed to acidic by intercalation of an inorganic compound, Cp2ZrCl2 showed the catalytic activity for ethylene polymerization in combination with the products of the partial hydrolysis of TMA on acidified Kunipia without methylaluminoxane (MAO). (C) 2003 Elsevier B.V. All rights reserved.