Sr 3 Fe 2؊x Co x O 7؊ oxides crystallizing in a tetragonal perovskiterelated intergrowth structure have been synthesized for 04x40.8 and characterized by X-ray powder di4raction, wetchemical analysis, and electrical and magnetic measurements. The lattice parameters, oxygen content, and average oxidation state of iron and cobalt decrease with increasing Co content x. However, the samples show instability on exposure to ambient air for prolonged periods of time due to a possible reaction with H 2 O and CO 2 . Resistivity measurements indicate that the samples exhibit a semiconducting behavior at low temperatures (804T4680 K) and the resistivity value decreases with increasing Co content. For T'680 K, the resistivity versus temperature plots exhibit a change in the sign of the slope from negative to positive due to oxygen loss at higher temperatures. Low-temperature resistivity data show an exponential temperature dependence of Jexp(T 0 /T ) with ؍ 1 2 for samples with 04x40.25 and ؍ 1 4 for samples with 0.64x40.8 suggesting a transition from activated carriers involving a coulomb gap to thermally activated hopping as the Co content increases. SQUID magnetometer measurements indicate a transition from antiferromagnetic behavior to weak ferromagnetic behavior as the Co content increases.