A thermodynamic and experimental investigation of a new class of solar thermochemical redox intermediates, namely, lanthanum-strontium-manganese perovskites, is presented. A defect model based on low-temperature oxygen non-stoichiometry data is formulated and extrapolated to higher temperatures more relevant to thermochemical redox cycles. Strontium contents of x = 0.3 (LSM30) and x = 0.4 (LSM40) in La1-xSrxMnO3-delta result in favourable reduction extents compared to ceria in the temperature range of 1523-1923 K. Oxidation with CO2 and H2O is not as thermodynamically favourable and largely dependent upon the oxidation concentration. The model is experimentally validated by O-2 non-stoichiometry measurements at high temperatures (>1623 K) and CO2 reduction cycles with commercially available LSM35. Theoretical solar-fuel energy conversion efficiencies for LSM40 and ceria redox cycles are 16 and 22% at 1800 K and 13 and 7% at 1600 K, respectively.