Nanostructured (3-6 nm) thin films (80 nm) of SnO 2 and Pt-doped SnO 2 were obtained by a new sol-gel route using tetra(tert-butoxy)tin(IV) and bis(acetylacetonato)platinum(II) as metal precursors. The results of glancing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS) investigations demonstrated that platinum substituted for tin(IV) in the cassiterite structure. Electron paramagnetic resonance (EPR) and XPS analyses showed that singly ionized paramagnetic oxygen vacancies (V O .) were formed on pure SnO 2 thin films by interaction with CO atmosphere; in Pt-doped SnO 2 films, the same defects V O . fully transferred their electrons to the noble metal so that Pt(IV) became Pt(II) and Pt(0). Such samples successively exposed to air, at room temperature, reduced O 2 to O 2 - . The behavior was well-detected by EPR measurements, which showed on thin films the presence of Sn(IV)-O 2 - species. The surface reactivity agrees with the results of the electrical measurements.