The influence of potassium doping on the oxygen-ion diffusion and the ionic conduction in the La2Mo2O9 oxide-ion conductors has been systematically investigated via the dielectric and mechanical relaxation techniques and the dc conductivity measurements. Two relaxation peaks are observed in the K-doped samples. It is found that the partial substitution of K+ ions at La3+ sites increases the activation energy of oxygen ion diffusion, which can be understood because K+ ions have a larger ionic radius in the crystal lattice and a greater chemical affinity with oxygen ions in comparison with La3+ ions. Based on the results of conductivity measurements, it can be concluded that K doping can effectively suppress the phase transition and the highest conductivity at lower temperatures is obtained in the 2.5% K-doped La2Mo2O9 samples. (C) 2004 Elsevier B.V. All rights reserved.