The crystal chemistry of a recently reported mixed conductor SrFeCo0.5Oy, exhibiting unusually high oxygen permeability, has been characterized by systematically investigating the Sr(Fe, Co)(1.5)O-y system. The samples belong to the general stoichiometry Sr4Fe6-xCoxO13+delta and have an orthorhombic, intergrowth structure isotypic with Sr4Fe6O13 for 0 less than or equal to x less than or equal to 1.5. While the intergrowth phase Sr4Fe6-xCoxO13+delta and the perovskite phase Sr(Fe, Co)O3-delta coexist along with CoO impurity for 1.7 less than or equal to x less than or equal to 2.1, the perovskite phase is formed along with CoO impurity for x = 2.4. in the single-phase region with 0 less than or equal to x less than or equal to 1.5, the oxygen content decreases, and the total conductivity increases with x. The occurrence of unusually high oxide-ion conductivity is explained on the basis of crystal chemistry.