We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeO x and Pt/CeSnO x for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnO x and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt 2+ and Pt 4+ ions in the catalyst Pt/CeO x , whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnO x . Insertion of Sn ions into the Pt/CeO x lattice destabilizes/reduces Pt 4+ cations in the Pt/CeSnO x catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce 3+ ions. Our DFT calculations corroborate destabilization of Pt 4+ ions by incorporation of cationic Sn in Pt/CeO x . The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions.