Separation of O-2 from air to produce a high-purity stream of N-2 is considered via a two-step solar thermochemical cycle based on aluminum-doped strontium ferrite reduction/oxidation reactions. The cycle steps encompass the (1) thermal reduction, driven by concentrated solar irradiation; and (2) re-oxidation with atmospheric air to produce a N-2 stream. Samples were synthesized via the sol-gel method, and crystalline structures were confirmed with x-ray powder diffractometry. Thermogravimetry was used to measure the nonstoichiometry at chemical equilibrium for molar B-site Al dopant fractions between 0 and 0.20, temperatures between 673 and 1373 K, and volumetric gas flow compositions between 1 and 90% O-2-Ar. The compound energy formalism was applied to thermogravimetric measurements to predict equilibrium nonstoichiometry as a function of dopant fraction, temperature, and O-2 partial pressure. The results were used to predict partial reaction enthalpies and entropies. Aluminum doping increased the strontium ferrite oxygen affinity but reduced the range of nonstoichiometries. A strontium ferrite sample underwent 10 thermal reduction-oxidation steps in an upward flow reactor coupled to a high-flux solar simulator to study cyclability. A thermodynamic cycle analysis was performed, and cycle efficiencies were determined relative to an ideal separation process. For a solar reactor at 1073 K, 0.59 mol N-2 per mol strontium ferrite could be produced at a purity of nearly 99% with a cycle efficiency of 4.0%.