The low-temperature oxidation of polycrystalline palladium by RF oxygen plasma was studied via X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Detailed information about the electronic states of palladium and oxygen was obtained based on the XPS curve fitting analysis of Pd3d and Pd3p + O1s lines. The results showed that Pd oxidation by oxygen plasma was different from Pd oxidation in pure O-2 at high temperature. SEM shows well-structured submicron PdO particles result from oxidation in pure O-2, whereas plasma oxidation results in the predominant formation of two-dimensional PdO structures covering the initial crystallites of the Pd foil. Further oxidation to a three-dimensional PdO phase occurs under prolonged treatment with oxygen plasma. The formation of a PdOx (x > 1) species, characterized by a E-b(Pd3d(5/2)) = 338.0-338.2 eV value that is close to the Pd4+ oxidation state, was also observed. This PdOx species was found to have low thermal stability (T < 400 K). It is proposed that the PdOx species can be localized within the boundaries of crystallites. (C) 2009 Elsevier B. V. All rights reserved.