The steady-state current-overpotential characteristics of the O-2,La0.6Sr0.4Co0.8Fe0.2O3-delta/YSZ interface have been studied as a function of oxygen partial pressure and temperature. Ideal Nernst behaviour is observed in the temperature range between 400-900 degrees C and oxygen pressure range between 0.5-100 kPa. The results of I-eta measurements indicate that in the potential range 0.05-0.25 V, the apparent anodic and cathodic charge transfer coefficients are close to unity : alpha(a) = alpha(c) = 1. The logarithm of the equilibrium exchange current density (I,) shows a positive dependence on the logarithm of the oxygen partial pressure with a slope m = 0.25+/-0.05. These observations are in agreement with a proposed reaction model in which the diffusion of singly ionized oxygen adatoms (O-ads(-)) on the oxide surface is assumed to be the rate determining step of the electrode reaction.