The current research investigates the phase separation phenomenon in (La0.8Sr0.2)0.95(CrxFe1-x)O3±δ (LSCrF) under the fabrication and processing conditions of Oxygen Transport Membranes (OTM). The research employs an integrated thermodynamics approach with the focus on changes in crystal structures and phase stability on Cr-rich perovskites. The chemical stability has been examined under high temperatures (700–1400 °C) with different oxygen partial pressures (p(O2)) (0.21 to 10−19 atm). The results reveal complicated correlations among dopant disintegrations in formations of secondary phases and the phase separation of the perovskite phase. The research provides predictions and practical procedures in the purpose of extending the lifetime of OTMs under processing conditions. The results for the perovskite phase have also been extended to examine the phase stability in dual-phase composites of Yttria-Stabilized ZrO2 (YSZ) and LSCrF for high-temperature ceramic membranes.