Two-step thermochemical redox cycles represent an attractive pathway toward producing solar fuels via H2O and CO2 splitting. These cycles typically operate at two different temperatures, but isothermal cycling has been proposed as a means to minimize the sensible heating requirements of the solid phase. Recently, a theoretical analysis showed that lanthanum-strontium-manganese-based perovskites compare favorably to the state-of-the-art material, ceria, when evaluated in the context of isothermal cycling. In this work, a high-temperature reactor system that affords precise control of partial pressures of H2O and H-2 (pH(2)O/pH(2)), and thus reduction and oxidation conditions, was utilized for isothermal cycling of ceria, La0.6Sr0.3MnO3 (LSM35) and La0.6Sr0.4MnO3 (LSM40), in order to validate theoretical predictions experimentally. Data are measured between 1300 and 1400 degrees C and between oxygen partial pressures (pO(2)) of 8 X 10(-7) atm (reduction, pH(2)O/pH(2) in the range of 62.73-203.17) and 5 X 10(-5) atm (oxidation, pH(2)O/pH(2) in the range of 250.92-507.93). Both LSM35 and LSM40 significantly outperformed ceria while operating isothermally in terms of H-2 production and performance (H), a new metric introduced in this article. This material-specific metric is related to efficiency, but enables straightforward comparison of materials directly from only a few measured experimental data points. Overall, experimental data agree well with theoretical predictions, giving promise to isothermal operation for this class of materials. Overall, this work provides motivation for material exploration and development of materials that undergo large changes in nonstoichiometry in the pO(2) range between 8 X 10(-7) atm and 5 X 10(-5) atm. Importantly, because operation is isothermal or nearisothermal, material exploration is not limited to materials possessing large entropic changes similar to those aimed specifically at temperature-swing redox cycling, opening the door to new possibilities and different material classes from those have been considered prior.