The oxygen stoichiometry of La1-xSrxCo1-yFeyO3-delta perovskite powders LSCF 8228, 6428 and 2882 (numbers refer to stoichiometry) was investigated by means of solid electrolyte coulometry (SEC) and temperature-programmed reduction using carbon monoxide (CO-TPR), Experiments were performed at reduced oxygen partial pressures between 1.3 x 10 (- 3) and 6 x 10 (-5) atm, applicable to cathodic bias under solid oxide fuel cell operating conditions. It was found that the degree of oxygen nonstoichiometry (delta) increased with increasing temperature, decreasing oxygen partial pressure and increasing Sr or Co content. CO-TPR studies indicate that the rate of CO2 production is dependent on the heating rate (beta), as well as the perovskite composition. In situ mass analysis during carbon monoxide oxidation indicates that the samples with low Sr or Co content (6428, 8228) show increased resistance to carbon deposition and/or carbonate formation.