The stability of La2Mo2O9-delta oxide-ion conductor was studied under Ar-H-2 and controlled oxygen partial pressure (pO(2)) atmospheres within the range 608 <= T <= 1000 degrees C, by thermogravimetry (TG), X-ray Diffraction (XRD), Temperature Controlled X-ray Diffraction (TCXRD) and Scanning Electron Microscopy (SEM). At 608 degrees C under Ar-H-2 flowing atmosphere, La2Mo2O9 phase was found to be unstable and reduction leads to an amorphous phase with composition close to La2Mo2O6.88. The stability of this amorphous phase under Ar-H-2 was analyzed by TCXRD and found to be stable below 900 degrees C. Isothermal TG under controlled pO(2) at 1000 degrees C indicated that La2Mo2O9-delta is not stable below 10(-7) Pa and decomposition took place without the formation of La7Mo7O30 or amorphization. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.