Oxygen nonstoichiometry and thermo-chemical stability of the perovskite mixed-conductors La0.6Sr0.4FeO3-δ and La0.6Sr0.4CoO3-δ were investigated by coulometric titration and high-temperature gravimetry in the oxygen partial pressure range of log(pO2/bar)=-28 to 0 at 773 to 1173 K. For both materials, the oxygen content decreased with decreasing oxygen partial pressure and increasing temperature. In the range log(pO2/bar)=-24 to -10, the oxygen content of La0.6Sr0.4FeO3-δ reached a plateau at δ=[Sr]/2=0.2, corresponding to stoichiometric composition. At lower oxygen partial pressure, the oxygen content further decreased and the material decomposed. La0.6Sr0.4CoO 3-δ was found to easily decompose already at higher oxygen partial pressure. The crystal structure of La0.6Sr 0.4FeO3-δ and La0.6Sr 0.4CoO3-δ was characterized by high-temperature X-ray diffraction in air as a function of temperature. Lattice parameters were calculated for both materials.