Electron energy loss spectra (ELS) and X-ray photoelectron spectroscopy (XPS) data obtained from palladium powder catalysts used for complete methane oxidation have been obtained in this study in order to gain information about the Pd chemical state. Metallic Pd and PdO yield significantly different ELS spectra making ELS useful for chemical-state determination. Palladium(II) oxide is readily detected by the presence of an energy loss feature at 3.7 eV Species distribution in the direction normal to the surface can be determined using ELS by varying the primary beam energy. Both XPS and ELS data indicate that PdO forms during reaction, and the ELS data demonstrate that PdO forms as a film over the I'd metal. If any Pd metal is present in the PdO film, it is there at levels below the detection limit of ELS. These observations have important consequences with regard to the mechanism of methane oxidation over Pd-containing catalysts. Hydrous and anhydrous palladium(II) oxides have significantly different energy loss features in the low loss-energy region of the ELS spectra. ELS features attributed to water are observed at 5.5-6.5 and 8 eV for hydrous palladium(II) oxide. (C) 2002 Elsevier Science B.V. All rights reserved.