The Pt oxidation state and nanostructure on a ceria-based oxide support were characterized after sequential oxidative, reductive, and re-oxidative treatments using X-ray photoelectron spectroscopy and transmission electron microscopy. The Pt oxidation state and nanostructure depended on the treatment atmosphere: an oxidized Pt monolayer spread along the primary particle surface of the ceria-based oxide support in the oxidized catalyst, whereas metallic Pt existed as particles of 1-2 nm in diameter in the reduced catalyst. Therefore, on the ceria-based oxide surface oxidized Pt formed a nanocomposite oxide with Ce under an oxidative atmosphere and reversibly changed to metallic Pt particles under a reductive atmosphere. The formation of this surface nanocomposite oxide is evidence of the strong interaction between oxidized Pt and CeO2 that involves the formation of a Pt-O-Ce bond, which stabilizes Pt against sintering and redisperses agglomerated Pt particles under an oxidative atmosphere. (C) 2009 Elsevier Inc. All rights reserved.