Here, our recent study on the oxygen-storage materials BaLnMn₂O_5+δ with Ln = La, Nd, Gd, and Y is reviewed. The redox characteristics of these materials were systematically investigated employing the reductive water dissolution by the deoxygenated δ = 0 form. The Ln = La, Nd, and Gd compounds were found to show a capability to produce hydrogen gas through the water dissolution at 500 ^oC, whereas the Ln = Y compound was unreactive to water. The present study also revealed that the reactivity significantly depends on the Ln species: the larger the Ln³⁺ ionic size, the higher reactivity the BaLnMn₂O_5.0 samples exhibit. The experimental data were discussed on the basis of thermodynamic energetics by quantum chemical calculations.