The behaviour of the SmCo0.8Ti0.2O3/Na5TbSi4O12 interface in argon and oxygen atmospheres at 250-300 degrees C was studied using the impedance spectroscopy method. An oxygen atmosphere is shown to affect the parameters of the relaxation impedance, which suggests a reversible character of this system with respect to oxygen. However, high values of the Warburg diffusion constant W = RT/(nF)(2)c(i)(2D(i))(1/2) imply severe diffusion control over the one-electron electrochemical reaction O-2(g) + e(-)double left right arrow O-2(-) determined by the potentiometric method.