The interaction of tin with cerium oxide thin film and cerium oxide nanosized powder was studied using XPS and resonant photoelectron spectroscopy (RPES) in the Ce 4d–4f photoabsorption region. A strong tin–ceria interaction led to partial Ce4+→ Ce3+ transition and Sn+ ion formation. A corresponding increase of the Ce 4f state occupancy was observed as a giant 4f resonance enhancement. In the case of powder catalyst a strong CeO2Sn interaction lead to formation of CeSnO mixed oxide system characterized by Ce3+ ion concentration enhancement and increase of Ce 4f occupation. It explains generally higher catalytic activity of the CeSnO mixed oxide catalysts when comparing with the individual oxides. Copyright © 2008 John Wiley & Sons, Ltd.