The real oxidation states of active sites on the important industrial Phillips type catalysts are still unclear. In this study, Phillips catalysts calcined at different temperatures (400, 600 and 800 degrees C) were modified by triethylaluminum(TEA) at different Al/Cr molar ratio and subsequently characterized by X-ray photoelectron spectroscopy (XPS) method and ethylene polymerization tests in order to elucidate the real oxidation states of active sites. Activity of each catalyst increases up to a maximum value then drops down with the increasing of Al/Cr molar ratio. The TEA-modified catalyst calcined at higher temperature got a larger maximum value of polymerization activity at a lower Al/Cr molar ratio. XPS method quantified the mixed Cr oxidation states including +2, +3, +5 and +6 of surface chromium species, in which only the +2 and +6 Cr species were found to be relating to the polymerization activity. The real active chromium precursor might be a chromium cluster (Scheme 1) named as Cr(2+.)2Cr(6+) composed of one Cr(II)O-x,O-surf species and two Cr(VI)O-x,O-surf species, in which the former acts as the real center of active Cr cluster and the latter acts as the necessary neighboring ligand environment. The relative concentration of active sites is estimated to be around 14.4-24.9 (mol % Cr) for the TEA-modified Phillips catalysts depending on the calcination temperature and Al/Cr molar ratio. (c) 2005 Elsevier B.V. All rights reserved.