La1-xSrxCo1-yFeyO3-delta perovskite materials have been considered as potential cathodes for solid oxide fuel cells operating at intermediate temperatures. The effect of temperature, ambient oxygen partial pressure and composition on the extent of oxygen deficiency was electrochemically investigated for various perovskites by means of solid electrolyte coulometry. Experiments were performed at reduced oxygen partial pressures between 1.3 X 10(-3) and 6 X 10(-5) atm since in a real fuel cell under cathodic bias, the material near the electrode/electrolyte interface is expected to be in a reduced state. It was found that the degree of oxygen non-stoichiometry (delta) increases with increasing temperature, decreasing oxygen partial pressure and increasing Sr or Co content. Transient re-equilibration experiments were also performed in an attempt to investigate the oxygen transport characteristics of the perovskite under various operating conditions. These experiments have shown that the time needed for re-equilibration increases with increasing oxygen deficiency.