In situ observation of oxygen absorption/release of ceria-zirconia solid solution with dispersed fine Pt particles was carried out by transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Oxygen absorption/release in TEM was enabled by controlling the degree of vacuum and the flux of electron beam. The oxygen absorption/release, Ce2Zr2O7 -> Ce2Zr2O7.5 -> Ce2Zr2O8, was confirmed by the analysis of TEM images and the change in the relative intensity ratio of Ce-M-4,M-5 white line peaks in EELS spectra, which corresponds to the valence change of Ce (Ce3+ reversible arrow Ce4+). The reversible oxygen absorption/release, Ce2Zr2O8 reversible arrow Ce2Zr2O7.5, also was found. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2130310] All rights reserved.