Electrical conductivity relaxation experiments were performed on thin specimens of (x = 0.1, 0.4) at oxygen partial pressures in the temperature range 923 to 1223 K. The transient response of the electrical conductivity after a sudden change of the ambient oxygen partial pressure was analyzed in the frequency domain. The latter procedure allowed diffusion‐limited and surface exchange‐limited kinetics of re‐equilibration to be distinguished. The response of specimens with thicknesses of 350 to 460 μm indicated diffusion‐controlled kinetics at . The chemical diffusion coefficients, D̃, were found invariant with oxygen pressure. At 1073 K the absolute values were for x = 0.1 and for x = 0.4, with activation energies of about 80 kJ/mol. The equilibration process was governed by surface exchange at . The surface exchange coefficient, k _O, was proportional to , where n = 0.65 to 0.85. This pressure dependency was interpreted in terms of a slow surface process involving an oxygen molecule and a surface oxygen vacancy, and causes the observed sharp transition from diffusion‐ to exchange‐controlled kinetics. The activation energy of k _O was estimated at 110 to 135 kJ/mol.