Oxygen nonstoichiometry and diffusivity are two key factors governing the oxygen transport of mixed-conducting oxides. Both properties are closely related to the structure and chemical composition. The oxygen nonstoichiometry can be derived from the oxidation states of the transition metals in the perovskites, whereas the oxygen diffusivity is reflected by the oxygen permeability. In this work, structural variations, transition metal oxidation states, and oxygen permeation fluxes of Ba(Co,Fe,Nb)O<SUB>3-δ</SUB> were investigated with XRD, XANES, and oxygen permeation measurements. Substitution of Fe for Co results in a smaller lattice constant, whereas Nb substitution leads to the opposite trend. The oxidation state of cobalt is smaller than 3+ and increases slightly as its content decreases; whereas the oxidation states of iron and niobium are close to 3+ and 5+, respectively, and are almost independent of the composition. Increasing the iron and niobium content leads to a diminished oxygen nonstoichiometry and oxygen permeability.