In this paper, the influence of cationic substitutions at Mo site with Al3+, Fe3+, Mn4+, Nb5+ and V5+ ions on the structure, oxygen ion diffusion and electrical properties in La2Mo2O9 oxide-ion conductors have been investigated by X-ray diffraction method, dielectric relaxation technique and direct current conductivity measurement. Except for V5+ substitution all of these substitutions up to 5% cannot suppress the phase transition in La2Mo2O9. In the dielectric measurement, one prominent relaxation peak is observed in temperature spectrum as well as in frequency spectrum, which is associated with the short-distance diffusion of oxygen vacancies. The activation energy for oxygen ion diffusion is deduced as in the range of 1-1.1 eV for Al, Fe, Mn and Nb doped samples and 1.4-1.5 eV for V doped samples. All substituted samples have a higher conductivity than the un-doped compound. In the Al, Fe, Mn and Nb substituted materials the phase transition is not suppressed; however, K substitution at the La site can completely suppress the transition and maintains high conductivity at low temperature. (C) 2006 Elsevier Ltd. All rights reserved.