Though reports in the literature have been interpreted as indicating that the presence of boundaries between submicron grains has the potential to dramatically increase the ionic conductivity of Gd-doped CeO2 (GDC) films [1-3], unambiguous interpretation is hampered by the lack of a study directly comparing the ionic conductivity of single-crystal GDC films to films that are identical except for the presence of submicron grain boundaries. Techniques are developed to grow GDC films by RF magnetron sputtering from a (10%)Gd2O3-(90%) CeO2 target onto a single crystal r-plane sapphire substrate that, though otherwise are largely identical, differ in that one film is a single crystal while the other is polycrystalline with similar to 80 nm diameter grains. The ionic conductivity of these films is measured in the temperature range of 400-700 degrees C in the Van der Pauw geometry. Analysis of the In(sigma T) vs. 1/T data reveals the single crystal and polycrystalline GDC thin films differ primarily in that the single crystal film exhibits a lower activation energy for ionic conduction of 0.85 +/- 0.01 eV than the 0.99 +/- 0.01 eV observed for ionic conduction in the polycrystalline film. The presence of 80 nm grains reduces the ionic conductivity of Gd-doped CeO2 in the temperature range of 400-700 degrees C. (C) 2010 Elsevier B.V. All rights reserved.