Lattice parameters, microstructures, thermal expansion coefficients and electrical conductivities of Ga-doped La0.6Sr0.4CoO3-y were investigated in order to characterize influences of interdiffusion of Ga from La(Sr)Ga(Mg)O3-y electrolyte into La0.6Sr0.4CoO3-y cathode in intermediate temperature solid oxide fuel cell. Unknown phases were formed when the amount of Ga doped was beyond 20% of B-site cation in the perovskite structure. In the single-phase region, the lattice parameters and thermal expansion coefficients were increased with the Ga doping. The thermal expansion coefficient increases were much higher than those of La(Sr)Ga(Mg)O3-y electrolyte. The electrical conductivities decreased with an increase in the doped amount of Ga, and the electrical conduction changed from metallic to semiconductor-like with the Ga doping. Those results clearly indicate that the interdiffusion of Ga into La0.6Sr0.4CoO3-y cathode has unfavorable influences on the physicochemical properties of La0.6Sr0.4CoO3-y in the intermediate temperature solid oxide fuel cell. (C) 2004 Elsevier B.V. All rights reserved.