Phase change characteristics in GexSb1-x films with various compositions (Ge20.3Sb79.7, Ge14.2Sb85.8, and Ge7.6Sb92.4) were investigated. A change in crystalline structure indicated that the phase separation occurred at temperatures that exceeded the phase transition temperatures of the films, and this change was dependant on the composition of GexSb1-x. The extended X-ray absorption fine structure data showed a change in the bond coordination of Ge from fourfold to threefold during the phase transition. Moreover, the phase separation of Ge occurred gradually with increasing temperature, due to the weak Ge-Sb bonds in the crystalline state. The Raman spectroscopy data showed a suppressed Peierls distortion caused by the Ge-Sb bonds in the crystalline state. The presence of weak Ge-Sb bonds results in instability of the GexSb1-x films, resulting in an irreversible transition. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3491738] All rights reserved.