The effect of aging and heating treatments up to 500 K on alumina-supported CuCl2, i.e., the base catalyst for ethylene oxychlorination, has been investigated by UV-vis spectroscopy, a solubility test, EXAFS, XRD, and EPR in a wide range (0.25-9 wt%) of Cu concentration. It is shown that the catalyst undergoes significant changes with both time and thermal treatments, so accounting for some contradictory results reported in the literature. While the surface Cu aluminate (formed during impregnation) does not change upon aging and heating, supported CuCl2 (precipitated from impregnating solution during the drying process) undergoes a slow hydrolysis reaction with the formation of paratacamite and HCl, The HCl formed during the hydrolysis reacts with the alumina surface with the formation of >Al-Cl species. Upon heating, the initially formed paratacamite can react with surface >Al-Cl species with nearly total restoration of CuCl2, which is consequently the main species present on the catalyst at the beginning of the oxychlorination reaction. The obtained picture is able to explain the results emerging from activity tests on the whole set of catalysts, indicating that surface aluminate is not active and that the active phase is CuCl2. (C) 2000 Academic Press.