Thermodynamical stability and electronic structure of a perovskite-type hydride, NaMgH 3, exhibiting reversible decomposition and recombination reactions, were studied experimentally. NaMgH 3 decomposes in two steps accompanying hydrogen desorption reactions, and the enthalpy change (Δ H) and the entropy change (Δ S) are calculated to be Δ H (1) = −93.9(6) kJ/mol H 2 and Δ S (1) = −116.2(9) J/K mol H 2 for the first step, and Δ H (2) = −102.2(4) kJ/mol H 2 and Δ S (2) = −125.9(6) J/K mol H 2 for the second step, respectively. Maximum entropy method (MEM)/Rietveld analysis of synchrotron X-ray diffraction profiles indicated that NaMgH 3 contains two hydrogen anions at the different occupation sites.