The oxygen nonstoichiometry of Ruddlesden-Popper compounds with chemical composition (RE2 − xSrx)0.98(Fe0.8Co0.2)1 − yMgyO4 − δ (RE = La, Pr, x = 0.9–1.2 and y = 0, 0.2) was measured as a function of temperature and oxygen activity (aO2) by coulometric titration and thermogravimetry. All compositions were found to be approximately stoichiometric in air (δ ≈ 0). The oxidation state of Fe and Co was determined by XANES. Fe retains an oxidation state of + 3 upon reduction of the sample, whereas Co is reduced to an oxidation state of + 2. A model of the defect chemistry is proposed that can account well for the measured oxygen activity dependence of the oxygen nonstoichiometry at all temperatures investigated. The studied compositions exhibit remarkable thermodynamic stability under reducing conditions. Decomposition was only observed for temperatures above 800 °C in a hydrogen water vapor gas mixture ([H2]/[H2O] = 50). The thermal and chemical expansion coefficients of these compounds are significantly decreased compared to those of (La0.6Sr0.4)0.99Fe0.8Co0.2O3 − δ, a well studied perovskite with related composition. The transport properties were investigated by conductivity relaxation and the potential of using these materials as oxygen separation membranes was assessed by calculating the oxygen flux that can be delivered through a 30 μm thick membrane.