BaO is a proven oxygen-selective sorbent for air separation via the BaO-BaO2 oxygen sorption-desorption cycle. However, the low transition O-2 pressure necessitates a high vacuum that is required for desorption, hence limiting its application. Here, we report facile synthesis of new oxygen-selective BaxSr1-xO/MgO nano-composite sorbents from which O-2 can be desorbed at moderate vacuum pressures. These sorbents are obtained from BaxSr1-xMg(CO3)(2) precursors synthesized by co-precipitation followed by calcination. It is shown that the transition O-2 pressure for the oxide-peroxide cycle is significantly increased by increasing the Sr2+/Ba2+ ratio. XRD results show that the crystal structures are preserved while the lattice constants are reduced upon substitution of Ba2+ by Sr2+ in both precursor carbonates and peroxides. Transmission electron microscopy/energy dispersive X-ray spectroscopy results show that Ba2+ and Sr2+ are mixed molecularly in BaxSr1-xO/MgO nanocomposite. We explain that the uniformly substituted Sr2+ in BaO/BaO2 changes the dissociation energy of peroxide ions and the equilibrium of the reversible reactions, promoting decomposition of BaO2. Cyclic O-2 sorption and desorption with BaxSr1-xO/MgO nano-composite is demonstrated. (C) 2015 Elsevier Ltd. All rights reserved.