Phillips CrOx/SiO2 catalyst is still producing several million tons of polyethylene each year. However, insufficient understanding of the surface chemistry of this important industrial catalyst hindered its further development. In this work, surface analytical approach on transformation of chromium(III) acetate into supported chromate species (Cr(VI)O-x,O-surf) of Phillips CrOx/SiO2 catalysts isothermally calcined at various temperatures was carried out by high resolution XPS method. High resolution XPS characterizations have elucidated the specific transformation process of chromium(III) acetate into bulky CrO3 and subsequently into Cr(VI)O-x,O-surf as a function of calcination temperature between 120 and 800degreesC. The specific surface components in various catalyst samples versus calcination temperature were clarified. It was evident that chromium(III) acetate could be very easily oxidized and decomposed into bulky CrO3 during 120degreesC isothermal drying process. The bulky CrO3 started to be transformed into supported chromate species at 200degreesC and could be completely stabilized on silica get surface as chromate species at 400degreesC. Significant amount of bulky pentavalent Cr oxide (Cr2O5 or (Cr2O7)(4-)) due to partial thermal decomposition of bulky CrO3 was only observed on 200degreesC-calcined sample due to the incomplete stabilization of bulky CrO3 into chromate species at 200degreesC. As the precursor of active site on calcined Phillips catalyst, the chromate species (Cr(VI)O-x,O-surf) was found to be gradually becoming more and more electron-deficient with increasing calcination temperature of the catalyst from 200 to 800degreesC. Only a slight thermal-induced partial reduction of chromate species into Cr(III)O-x,O-surf was observed at high temperature (600-800degreesC). (C) 2004 Elsevier B.V. All rights reserved.