Mixing effects induced in CeO2/TiO2 and CeO2/SiO2 composite powdered materials by bombardment with Ar+ ions of 3.5 keV of kinetic energy have been studied by x-ray photoelectron spectroscopy (XPS) and the factor analysis method. Characterization of the ceria phase in the original composite samples was carried out by x-ray diffraction (XRD) temperature programmed reaction, transmission electron microscopy, and extended x-ray absorption fine structure. The CeO2/SiO2 sample is characterized by very small domains (∼25 Å in size) where the cerium has entered the Si–O lattice forming a kind of silicatelike amorphous phase. In CeO2/TiO2 the cerium is in a bimodal distribution with small particles of CeO2 (30–100 Å in size) and a cerium oxide phase formed by very small particles (d=16 Å) well dispersed on the TiO2 grains. XPS showed that either by heating in H2 or by Ar+ sputtering a Ce4+→Ce3+ reduction occurs to an extent which depends on the sample and reduction conditions. The reduction degree, determined from the XPS spectra by the factor analysis method, decreased in the sense CeO2/SiO2≳CeO2/TiO2≳ CeO2, for each sample being always higher after Ar+ sputtering than after heating in H2. Adsorption of O2 at 373 K on the reduced samples led to their partial reoxidation, to an extent which was dependent on the structure of the cerium in the samples and on the type of the previous reduction treatment. In the case of the CeO2/TiO2 sample the high reduction degree obtained by Ar+ sputtering can be explained by assuming a mixing of the cerium and titanium oxide phases induced by the ion bombardment. The use of a chemical probe such as the adsorption of O2 is proposed as a method to detect the existence of mixing phenomena in composite systems submitted to ion bombardment.