At high temperatures in reducing atmospheres, significant deficits of Ga have been observed on the surface of the doped LaGaO3 electrolytes, which can be attributed to the high partial pressure of Ga oxides such as Ga2O. The addition of Sr to the La sites accelerated the Ga depletion, whereas the addition of Mg to Ga sites did not. An attempt has been made to evaluate the apparent diffusion coefficients (D) and evaporation rate constants (alpha) from the Ga concentration profiles measured by SIMS on the La0.9Sr0.1Ga0.8Mg0.2O2.85 electrolyte which was annealed in H-2 - 1.2% H2O atmosphere for 10 h. The values of D (cm(2)/s) and alpha (cm/s) below 1223 K can be expressed as a function of temperature as follows: D = 6.3 X 10(-9) exp(-1.8 x 10(4)/T), alpha =7.9 x 10(3) exp(-3.5 x 10(4)/T) alpha steeply decreases with decreasing the operating temperature, and the temperature dependence of alpha is larger than that of D. (C) 2000 Elsevier Science B.V. All rights reserved.