The chemical diffusion coefficient in Ce0.8Gd0.2O1.9-delta was estimated by analyzing the weight relaxation behavior after an abrupt change of the oxygen partial pressure. The weight relaxation data were analyzed by fitting the data to solutions of Fick's second law for appropriate boundary conditions. The diffusion equation, which ignores the effect of surface reaction, failed to describe the transient behavior especially for the first stage. Taking the surface effect into account, the fitting gave a satisfactory interpretation of the overall relaxation process and allowed a precise determination of the chemical diffusion coefficient and surface reaction rate constant. The chemical diffusion coefficient was found to decrease with the decrease of the oxygen partial pressure, while the surface reaction rate constant increased with the decrease of the oxygen partial pressure. The increase of the surface reaction rate constant may be related to the increase of the oxygen nonstoichiometry, implying that the oxygen vacancies play an important role in the surface reaction kinetics. The ionic mobility was estimated from the obtained chemical diffusion coefficients, with the help of oxygen nonstoichiometry and electrical conductivity data obtained before.