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 Sr(4)Fe(6-x)CoxO(13 +/-delta) 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.