The oxidation state of the cerium in ceria (CeO2) nanocrystalline films electrodeposited on Si(001) substrate was probed by three different complementary techniques: X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES) and electron energy loss spectroscopy (EELS). CeO2 films were prepared under different oxygenation conditions, and subsequently thermal annealed as well as submitted to 30 and 350 keV Ne+ irradiation with fluences ranging from 2 x 10(14) to 2 x 10(16) ions/cm(2). Trivalent cerium concentrations higher than 50% were reached by using 30 keV Ne+ irradiation with high fluencies. Strong valence deviations in the film volume with specimen regions practically fully-reduced to CeO1.5 were obtained. These findings points out the potential for applications of low-cost nanocrystalline ceria thin films in many kinds of devices. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.056201jes].