The oxide-ion conduction properties of the series La2Mo2-yWyO9 (y <= 1.4) are studied by impedance spectroscopy. For each studied composition (y = 0.25, 0.5, 0.75, 1.0, 1.2 and 1.4) two conduction regimes are evidenced: a low temperature regime of conventional Arrhenius-type, and a high temperature regime interpreted as VTF (Vogel-Tammann-Fulcher)-type, with highly mobile oxide-ions. While the characteristics of the Arrhenius-type regime evolve regularly with tungsten substitution, those of the VTF-type regime vary non linearly, with an extremum, at y = 1. Both findings are analyzed in connection with similar trends previously evidenced in a crystallographic study [G. Corbel, Y Laligant, F. Goutenoire, E. Suard, P. Lacorre, Chem. Mater. 17 (2005) 4678], namely smooth evolution of O2/O3 sites occupancy, and non linear evolution of the cell volume and cationic framework characteristics, respectively. The best performance in term of conduction stability in a reducing atmosphere is obtained for doubly substituted La1.7Gd0.3Mo0.8W1.2O9 (stable at 696 degrees C down to P(O-2)= 1.6 10(-16) Pa). (c) 2006 Elsevier B.V. All rights reserved.