The compatibility of La0.9Sr0.1Ga0.8Mg0.2O2.85 perovskite oxide as an electrolyte for solid oxide fuel cells (SOFCs) was (i)nvestigated. After single SOFC tests using Pt as cathode and anode, the surface morphologies of La0.9Sr0.1Ga0.8Mg0.2O2.85 were changed. For the cathode side, second phases were formed along grain boundaries, but there was no significant change on the surface of grains. As for the anode side, two significant compositional changes were found by SIMS. The first is that La0.9Sr0.1Ga0.8Mg0.2O2.85 reacted with other components such as alumina tubes or Pyrex glass sealant in the test cell. The second is a significant depletion of B-site ions, especially Ga ion, from the surface outside the effective electrode area in La0.9Sr0.1Ga0.8Mg0.2O2.85, and the surface morphology was changed from dense to porous. Because Ga monovalent compounds, such as Ga2O, have high vapor pressures in the reducing atmosphere around p(O-2) = 10(-24) atm, the Ga depletion can be ascribed to the vaporization of the Ga component.