The incorporation of molybdenum in the Ruddlesden-Popper type Sr 3 Fe 2-x Mo x O 7-δ (x=0–0.1) decreases oxygen deficiency, thermal expansion and electron-hole transport, and increases n-type electronic conductivity in reducing atmospheres. The oxygen ionic conduction remains essentially unaffected by doping. The equilibrium p(O 2 )–T–δ diagram of Sr 3 Fe 1.9 Mo 0.1 O 7-δ , collected in oxygen partial pressure ranges from 10 −20 to 0.7atm at 973–1223K, can be adequately described by a defect model accounting for the energetic nonequivalence of apical O1 and equatorial O3 sites in the layered structure, in combination with iron disproportionation and stable octahedral coordination of Mo 6+ and Mo 5+ cations. The calculated enthalpy of anion exchange between the O1 and O3 positions, 0.49–0.51eV, is in agreement with the values predicted by the atomistic computer simulation technique. The high-temperature X-ray diffraction studies showed a strongly anisotropic expansion of the Ruddlesden-Popper lattice on reduction, leading to very low chemical strains favorable for electrochemical applications. At 298–1223K and oxygen pressures from 10 −8 to 0.21atm, the linear thermal expansion coefficient of Sr 3 Fe 1.9 Mo 0.1 O 7-δ varies in the narrow range (12.9–14.2)×10 −6 K −1 . The relatively low level of n-type electronic conductivity leads, however, to a poor performance of porous Sr 3 Fe 1.9 Mo 0.1 O 7-δ anodes in contact with lanthanum gallate-based solid electrolyte under reducing conditions.