The crystal chemistry and oxygen permeation properties of the mixed oxide-ion and electronic conductor LaSr3Fe3-xCoxO10 with 0 less than or equal to x less than or equal to 1.5 have been investigated. LaSr3Fe3-xCoxO10 has a perovskite-related intergrowth structure belonging to the n = 3 member of the Ruddlesden-Popper series and the tetragonal cell parameters and volume decrease with increasing Co content. The LaSr3Fe3-xCoxO10 +/- delta samples have excess (delta approximate to 0.1) oxygen at room temperature, but significant amount of oxygen vacancies (0.3 less than or equal to delta less than or equal to 0.7) at high temperatures (900 degreesC). The LaSr3Fe3-xCoxO10 samples are semiconducting with the total conductivity increasing with increasing Co content. However, above a transition temperature T > T-t approximate to 400 degreesC, the conductivity decreases with increasing temperature due to the loss of oxygen. The oxygen permeation flux across the LaSr3Fe3-xCoxO10 membranes at high temperatures (800 degreesC less than or equal to T less than or equal to 950 degreesC) increases with increasing Co content and the flux values are stable with time in the oxygen partial pressure range 10(-4) less than or equal to pO(2) less than or equal to 0.21 atm.