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Thermogravimetric analysis (TGA) technique in controlled oxygen partial pressure (pO(2)) atmospheres has been used to obtain equilibrium oxygen content data as a function of pO(2) on the La(2)Mo(2)O(9-δ) system resulting from the partial reduction of fast oxide-ion conductor La(2)Mo(2)O(9) (LM). Thermodynamic conditions for stabilization of crystalline La(7)Mo(7)O(30) and amorphous La(2)Mo(2)O(7-y) at 718 °C have been determined and discussed. At 608 °C, the compound reported for the first time La(2)Mo(2)O(8.96) (LM896) has been found. The crystalline form and transition temperature in LM896 have been identified by X-ray diffraction at room temperature (XRD) and at controlled temperature. Conductivity curves obtained by electrochemical impedance spectroscopy (EIS) as a function of temperature for both LM and LM896 have been compared. The results indicate that LM896 is a mixed ionic and electronic conductor (MIEC).