Electrolyte materials with composition La2Mo2-yWyO9 (y=0-1.5) exhibit high ionic conductivity ranging from 0.11 S cm(-1) (y=0) to 0.05 S cm(-1) (y=1.5) at 1023 K, which is comparable to gadolinia doped ceria. The ionic conductivity is predominant in a wide range of oxygen partial pressures from 0.21 to 10(-20) atm at 973 K with ionic transport numbers higher than 0.95 under humidified 5%H-2-Ar. Above this temperature, a significant increase of the n-type electronic conductivity and degradation of the ceramic microstructure are observed as a consequence of the cell 14 volume expansion in addition to the formation of new phases upon reduction. Considering the potential use of these materials as solid electrolytes, the chemical compatibility with most of the typical electrodes commonly used in SOFCs (e.g. cobaltites, ferrites, chromites and manganites) was investigated. Severe chemical reaction was found between La2Mo2-yWyO9 and most of the studied electrodes, mainly due to molybdenum migration. Furthermore, the high thermal expansion coefficients of La2Mo2O9 based materials (similar to 15 center dot 10(-6) and similar to 20 center dot 10(-6) K-1 in the low and high temperature range, respectively) restrict the choice of compatible materials as cell components. Moreover, electrolyte and electrodes present poor contact due to the low fixing temperature, necessary to prevent excessive chemical reaction between the materials, and also the large thermal expansion coefficients of lanthanum molybdate materials, resulting in the separation of both materials after several consecutive thermal cycles. Therefore, the use of these materials as SOFC electrolyte seems to be rather limited. (C) 2007 Elsevier B.V All rights reserved.