The chemomechanical and electrical properties of (Pr,Ce)O2-delta thin films were studied between 30 and 875 degrees C in air by in situ X-ray diffraction and complex impedance spectroscopy measurements. Reduction/oxidation reactions produced large stress variations (similar to 2 GPa) in the structure. Atomistic simulation techniques were employed to investigate the mechanisms behind the observed chemical expansion behavior, suggesting the possible roles of defect ordering upon cooling and heating. An alternative explanation based on a metastable frozen in state of higher reduction is also considered. Similar phenonena, observed here in Pr0.2Ce0.8O2-delta, are expected to be applicable and of potential significance for other technologically important complex oxides such as perovskite-structured materials. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.016212jes] All rights reserved.