The electrical properties of reduced LAMOX-type oxides (La1.9Y0.1Mo2-yWyO9-delta with y = 0, y = 0.5, y = 1.0) were investigated by complex impedance spectroscopy. When reduced at 605 degrees C in hydrogen, La1.9Y0.1Mo2O9-delta is 10 times and 3 x 10(5) times more conductive at 605 and 180 degrees C, respectively, than in air at the same temperatures. The conductivity curve presents a low slope (0.37 eV versus 1.2 eV in air). Besides, the stabilising effect of tungsten against reduction is evidenced, in good agreement with previous reports. In low oxygen partial pressures however (PO2 < 10(-18) Pa), the decomposition of the materials is detected, whatever the tungsten content (0 <= y <= 1 in La1.9Y0.1Mo2-yWyO9-delta). This observation points out the efficiency limit of Mo6+/W6+ substitution to stabilise the structure against reduction, and the limit for an application as IT-SOFC electrolyte. However, given the high electronic conductivity upon reduction, the application of these materials in IT-SOFC electrodes could be considered. (c) 2006 Elsevier Ltd. All rights reserved.