The electrical conductivity of SrFe0.67Ta0.33O3 − δ measured in the oxygen partial pressure range from 10− 20 to 0.5 atm at 700–950 °C, exhibits an anomalous and reversible increase in moderately reducing atmospheres, substantially contributed by oxygen ion transport. The coulometric titration studies in combination with the electron and X-ray diffraction analyses show that this trend correlates with the separation of oxygen-deficient disordered cubic perovskite and Ta-rich double perovskite domains of nanometer-scale size. Mössbauer spectroscopy demonstrated that the predominant state of iron cations in the vicinity of this transition is trivalent. Further reduction leads to the formation of Fe2 + states responsible for increasing n-type electronic conductivity, whilst no traces of wustite or oxygen vacancy-ordered brownmillerite phases were detected. The ion conduction in partially reduced SrFe0.67Ta0.33O3 − δ is significantly higher compared to SrFe0.7Ta0.3O3 − δ and SrFe0.6Ta0.4O3 − δ under similar conditions, which may indicate an important role of the interfacial boundary enlarged due to the perovskite phase separation.