The effect of the calcination temperature of alumina, which was then used as a support for a titanium halide catalyst [TiCl4/Et2AlCl], on the catalyst activity in ethylene polymerization was investigated. alpha-Al2O3 was found to make a more advantageous catalyst support as compared to gamma-Al2O3 despite the fact that the former offered a clearly lower specific surface area and its content of surface OH groups was inferior. The ethylene polymerization in the presence of the catalytic system on different alumina supports was investigated on the basis of a proposed kinetic model, taking into consideration the deactivation of active sites in the process. The improved activity was found to result from the better utilisation of adsorbed titanium chloride in the formation of active sites (increase of [C-0(*)]) and from the higher stability of said sites (lower k(t)), while their chemical nature underwent no significant change (k(p) and k(u) remain unchanged). (C) 2000 Elsevier Science Ltd. All rights reserved.