This paper reports transient weight curves of La0.2Sr0.8CoO3-delta (LSC) membrane (crashed in small grains) subject to an instantaneous change of oxygen activity in the gas phase. When the surrounding gas changes between pure oxygen and helium, the LSC sample experiences a slow weight loss in the oxygen desorption period and a rapid weight gain in the oxygen absorption period due mainly to the difference in the driving force. The LSC sample undergoes phase transformation/chemical reaction with the surrounding gas changing between the pure oxygen and methane. A mathematical model considering surface reaction as the rate-limiting step is presented to describe the oxygen transport through the membrane material, and employed to obtain the surface reaction rate constants. A simple correlation between the oxygen permeation flux and rate constants measured by the transient thermogravimetric analysis (TGA) method is given. The oxygen permeation fluxes calculated from the data measured by the present TGA method are consistent with the data measured by the permeation method. The TGA method is found particularly useful for studying the surface reaction rates of oxygen permeation through thin mixed-conducting ceramic membranes.