Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and zirconia (ZrO(2)) (111)-buffered sapphire (Al(2)O(3)) (0001) substrates using oxygen plasma-assisted molecular beam epitaxy to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on sapphire substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al(2)O(3) (0001) substrate and the ceria films. In contrast, the films, grown on a thin layer of ZrO(2) (111) buffered sapphire substrate, appear to be highly oriented in nature with predominant double domain (111) orientation. Oxygen ionic conductivity of these gadolinia-doped ceria films was measured as a function of gadolinium concentration and found to be efficient at relatively lower temperature operation compared to that of bulk polycrystalline, single crystalline yttria stabilized zirconia and gadolinia-doped polycrystalline ceria. Relative improvement in ionic conductivity of highly oriented gadolinia-doped ceria films (in the lower temperature regime) can be ascribed to the increased oxygen vacancies due to presence of Gd as well as high quality of the oriented thin films. (C) 2007 Elsevier B.V. All rights reserved.