Atomistic simulations of the structures and defect energetics of two mixed-conducting strontium ferrite materials, Sr₂Fe₂O₅ and Sr₄Fe₆O₁₃, are reported. Oxygen Frenkel defects are found to be the predominant intrinsic defects in both materials, with the Frenkel energy in the intergrowth structure, Sr₄Fe₆O₁₃, being much less than that in Sr₂Fe₂O₅ or other known oxide ion conductors. Formation of electronic defects under oxidizing and reducing conditions is calculated to be more favourable in the Sr₄Fe₆O₁₃ intergrowth structure than in Sr₂Fe₂O₅. Comparison of solution energies for cobalt incorporation shows Sr₄Fe₆O₁₃ has a slight preference for Co²⁺ being located on lower coordination sites, whereas in Sr₂Fe₂O₅, Co²⁺ ions located on tetrahedral sites are most favourable. Binding energy calculations suggest the possible formation of Co²⁺–vacancy clusters with increasing Co²⁺ concentration. The rapid oxide ion conductivity in Sr₄Fe_6−xCo_xO_13±δ membranes is thought to arise from a combination of factors: a low oxygen Frenkel energy (to produce an intrinsic population of mobile interstitial oxide ions and vacancies), low migration energy barriers, and ease of distortion of polyhedra.