Chemical Stability of La0.9Sr0.1Ga0.8Mg0.2O2.85 as an SOFC electrolyte was investigated in humidified hydrogen at 1273 K, After annealing at 1273 K, significant changes were observed in the surface morphology of the electrolyte, and La(OH)(3), LaSrGaO4 and some unknown phases were formed, which can be interpreted as a result of the vaporization of Ga components such as Ga2O. After annealing at 1073 K, changes in the morphology were small, but a slight amount of Ga depletion was also observed. The existence of Pt enhances the Ga depletion from the electrolyte in the reducing atmosphere probably because of the formation of Pt-Ga alloy.