Increasing temperature from 973 to 1173 K leads to a substantial increase of the electronic contribution to the total conductivity of undoped lanthanum molybdate and La2Mo2O9-based solid electrolytes, including La2Mo1.7W0.3O9, La2Mo1.95V0.05O9 and La1.7Bi0.3Mo2O9, where the stabilization of beta-La2Mo2O9 down to room temperature was confirmed by high-resolution X-ray diffraction (XRD) and differential scanning calorimetry (DSC) data. In air, the ion transference numbers determined by the modified Faradaic efficiency (FE) technique, decrease from 0.991-0.997 at 973-1023 K down to 0.977-0.984 at 1173 K. Reducing oxygen partial pressure also increases electronic conduction evaluated by the emf and oxygen permeability (OP) measurements, which indicates that the electronic transport is n-type, resulting front decreasing oxygen content in the molybdate lattice. The level of n-type electronic conductivity in air is quite similar for all La2Mo2O9-based ceramics. The results show that these materials can be used as solid electrolytes only under oxidizing conditions and only at temperatures below 1073 K. Their practical applications may also be complicated due to relatively high thermal expansion coefficients (CTEs), (14.1-14.8) x 10(-6) K-1 at 300-700 K and (16.4-22.5) x 10(-6) K-1 at 850-1070 K, which are close to those of stabilized delta-Bi2O3 and gamma-Bi2VO5.5 electrolytes. (C) 2004 Elsevier Ltd. All rights reserved.