The structural stability and ion conductivity of (La1.8Dy0.2)(Mo2-xWx,)O-9 are studied using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and impedance spectroscopy. The structural stabilization effect of W is demonstrated by comparing the microstructures and the oxidation states of ions of as-sintered and H-2-reduced samples. Without tungsten, the H-2-reduced surface of (La1.8Dy0.2) Mo2O9 is engraved with deep notches at grain boundaries and shallow cuts in the grain interior. At a sufficient W level, the H-2-reduced surface is similar to the surface without H-2 reduction. The XPS analysis concludes that 20% Mo on the (La1.8Dy0.2)Mo2O9 surface is reduced to Mo4+ and Mo-0, whilst all Mo is detected at the oxidation state of +6 in (La1.8Dy0.2)(Mo1W1)O-9 after 600 degrees C 3%H-2 reduction. No alpha-beta phase transformation is experienced in any specimens of (La1.8Dy0.2)(Mo-2-W-x(x))O-9, when heated from 300 to 800 degrees C. The room-temperature lattice parameter of beta-phase increases with increasing W content, reaches a maximum at x=1.0, then decreases. Considering the W structural stabilization effect, its unfavorable influence on conductivity is tolerable because the sample of (La1.8Dy0.2)(Mo1W1)O-9 exhibits a conductivity of 0.18 S cm(-1) at 800 degrees C, still higher than 0.08 S cm(-1) of La2Mo2O9. The temperature dependence of ion conductivity in this doubly substituted LAMOX is correlated to the Arrhenius form from 350 to 450 degrees C and the Vogel-Tamman-Fulcher form from 450 to 800 degrees C, and discussed. (c) 2007 Elsevier B.V All rights reserved.