The CO2 absorption properties and the microstructure of (Ba,Ca)(Fe,Mg)O3- have been studied by TGA, XRD, and Mössbauer spectrometry. Paramagnetic doublets of FeIV and FeIII appeared in the Mössbauer spectra of cubic (Ba0.5Ca0.5)(Fe0.5Mg0.5)O3- heated in CO2 up to 600 °C, and a pair of sextets of tetrahedral FeIII (Hin = 43 T) and octahedral FeIII (Hin = 51 T) were produced above 800 °C, and an additional sextet characteristic of FeIII in a spinel structure (Hin = 48 T) was observed at 1000 °C. On the other hand, a pair of sextets of tetrahedral and octahedral FeIII of the orthorhombic (Ca0.95Ba0.05)(Fe0.5Mg0.5)O3- showed hardly any change after absorption of CO2. It is concluded that only a small portion of Mg entered the orthorhombic phase of (Ca0.95Ba0.05)(Fe,Mg)O3- and Mg preferred the octahedral B site of the perovskite lattice. The excess Mg formed separate CaO-MgO mixed oxide, and the primary mechanism of CO2-trapping is the formation of CaMg(CO3)2.