Composition-tunable single-phase three-component alkaline earth oxide of (BaSrMg)O was prepared based on the consecutive precipitation and thermal decomposition of (BaSrMg)CO3. First, the single-phase (BaSrMg)CO3 was coprecipitated via ion self-assembly, phase-transformation, and agglomeration. The element composition of (BaSrMg)CO3 could be simply tuned by the composition of the reactants. Then, (BaSrMg)CO3 was converted to (BaSrMg)O under an H2 atmosphere at 750 °C. This (BaSrMg)O showed fast chemisorption-desorption responses with oxygen chemisorption rate: t80 = 3.9 min and desorption rate: t80 = 14 min and a high thermal stability for the redox reaction of BaO-BaO2. In addition, the chemisorption capacity of (BaSrMg)O (4.39% Sr composition) is ∼1.92 mmol/g, which is much higher than the chemisorption capacity of BaO/MgO at 1.75 mmol/g (Jin et al., Ind. Eng. Chem. Res., 2005, 44, 2942), while the transient oxygen pressure for the redox reaction of (BaSrMg)O (4.39% Sr composition) was significantly enhanced from 76 to 135 mmHg due to the inclusion of Sr in (BaSrMg)O. The transient oxygen pressure could be further improved via adjusting the Sr composition in (BaSrMg)O. Consequently, the tunable (BaSrMg)O has a high potential as a chemisorbent for the industrial application of oxygen separation.