Cobalt-substituted bismuth vanadates with composition Bi4V2-xCoxO11-delta (x = 0.0-0.2) were synthesized in single-and polycrystalline forms. It was confirmed that 10 at.% (x = 0.2) substitution of cobalt for vanadium stabilized the high temperature gamma-phase, which exibits a very high oxide ion conductivity, to room temperature by X-ray diffraction and differential thermal analysis. Results of X-ray photoelectron spectroscopy study revealed that there was no change in valence states of Bi and V due to the Co substitution; 3+ for Bi and 5+ for V, respectively. Conductivity measurements on single crystalline samples confirmed a high anisotropy of the ionic conductivity between the directions parallel and perpendicular to (Bi2O2)(2+) layer. 10 at.% substitution of cobalt for vanidium enhanced the oxide ion conductivity in the direction parallel to the (Bi2O2)(2+) layer (ab-direction), more than 3 orders of magnitude below 400 degrees C for single-crystalline samples, while 1.5 orders of magnitude for the polycrystalline sample.