Phillips CrOx/SiO2 catalyst is still an important industrial catalyst for ethylene polymerization. Poor understanding of the electronic states of surface chromate species has hindered further mechanistic exploration and improvement of this catalyst system. The objective of this work is to systematically study the photo-stability of Phillips catalyst by X-ray photoelectron spectroscopy (XPS) method, since such photostability has not been well documented yet in the literature. Five catalysts: namely, PC200, PC300, PC400, PC600 and PCSW with 1.0 wt.% of Cr loading were prepared from the industrial Phillips catalyst precursor PCP120 at 200, 300, 400, 600 and 800degreesC, respectively. The photostability in terms of electronic stability for the surface chromate species (Cr(VI)Q(x),(surf)) on these catalysts was investigated by a high resolution XPS method. The resistance to photo-reduction into supported Cr(III)O-x,(surf) species induced by X-ray irradiation during XPS measurement was studied. The photo-stability of surface chromate species on Phillips catalyst was found to be significantly dependent on the calcination temperature for preparation of the catalysts. A correlation between the photo-stability of surface chromate species and the Calcination temperatures for catalyst preparation has been established. Deeper insight on the electronic states of surface chromate species on Phillips type catalysts has been achieved by high resolution XPS methods. (C) 2004 Elsevier B.V. All rights reserved.