<p>The oxygen content in BaFe_1−<i>x</i>Ce_<i>x</i>O_{3−<i>δ</i>} (<i>x</i> = 0.1, 0.15) was measured by a solid state coulometric titration in the range of oxygen partial pressure between 10⁻²⁰ and 0.5&#xa0;atm at temperatures 750–950&#xa0;°C. The data obtained were used to model the defect chemistry in the oxides. It was found that cerium in BaFe_1−<i>x</i>Ce_<i>x</i>O_{3−<i>δ</i>} exhibits unconventional redox behavior. Good agreement between calculations and measurements was achieved under the assumption that only a fraction of cerium ions is reducible and that this fraction gradually decreases with increasing the overall cerium content. The unusual redox behavior is believed to be the result of a size effect. The Ce⁴⁺ ion is notably larger than the Fe⁴⁺ and Fe³⁺ ions. Furthermore, the reduction of cerium to Ce³⁺ is accompanied by a significant increase in ion size. However, the lattice stresses should counteract this process, thereby restricting the percentage of reducible cerium. The calculated concentrations of iron and cerium ions in different oxidation states were used to analyze the electrical conductivity of these oxides. It was found that cerium ions do not participate in electron transport in BaFe_1−<i>x</i>Ce_<i>x</i>O_{3−<i>δ</i>}. An increase in cerium content was shown to decrease the mobility of electrons and holes while increasing the mobility of oxygen ions.</p>