This paper aims at providing a better understanding of the electrical properties and mechanical behavior in reducing atmosphere (Ar-H-2) of La2Mo2O9-type ceramics in order to clarify their potential use as either SOFC electrolyte or anode. A careful analysis of the available literature on this topic is provided. The electrical properties, crystal structure and cerarnic microstructure of reduced La1.9Y0.1Mo1.5W0.5O9 are investigated by AC Impedance Spectroscopy (IS), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-Ray diffraction (XRD). At 400 degrees C, La1.9Y0.1Mo1.5W0.5O9 is metastable in 10% Ar-H-2. The reduction yields an electronic conductivity and results in a disappearance of all the electrical phenomena of interfacial nature. On severe reduction, the mass loss and particles ejection lead to an important contraction of the grains yielding strong damages at grain boundaries, where reduced phases and/or metallic molybdenum segregate, explaining the high electronic conductivity. (c) 2007 Elsevier B.V. All fights reserved.