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This paper reports the evaluation of the cubic B site cation-ordered double perovskite system Ba2Co2−x (Mo1/2Nb1/2) x O6, resulting in the single phase composition Ba2CoMo0.5Nb0.5O6−δ (BCMN) with a mixed Co charge state stabilized by a combination of Mo and Nb doping as a new mixed conductor with potential SOFC cathode applications. X-ray, neutron, and electron diffraction show that hexagonal intergrowths found in multiple phase samples at lower Mo/Nb contents are suppressed in BCMN, which has large domains of rock-salt ordered B site cations with separate Co and Mo/Nb sites. Conductivity measurement and impedance spectroscopy investigation shows that BCMN has a considerably reduced dc conductivity compared with materials such as BSCF but exhibits comparable electrochemical properties to some existing cathode materials in symmetrical cell measurements, and shows higher structural stability and reduced reactivity with the Ce0.8Sm0.2O2−δ (SDC) electrolyte. The role of Mo in dioxygen activation is proposed to offset the reduction that the d 0 Mo(VI) and Nb(V) cations on the B site produce in the electronic and ionic conductivity.