Support effect on the oxidation state of Pt nanoparticles was investigated by in situ laboratory XANES and O(2)-TPD The support oxides used were MgO and Al(2)O(3) In oxidizing condition, the surface of Pt nanoparticles supported on Al(2)O(3) was rapidly oxidized to become the oxidation state of Pt(s)(2)O to Pt(s)O stoichiometry, where Pt(s) stands for the surface Pt atom, but further oxidation was not observed even at high temperature. On the other hand. Pt supported on MgO was highly oxidized in two steps at high temperature; rapid oxidation exceeding the stoichiometry of Pt(s)O, and slow further oxidation toward the stoichiometry of PtO(2) The Pt particles on MgO would be redispersed and stabilized through the formation of atomically dispersed platinate anions, which would be tetravalent Pt cations coordinated by some oxygen anions, by the influence of the electrophobic cation of the support at high temperature and through the formation of Pt oxide cluster anions, (PtOx)(n)(y-), where x would be less than two, at lower moderate temperature (C) 2010 Elsevier B V All rights reserved