Literature reports suggest that the surface and interface regions of thin film Gd-doped CeO2 (GDC) may act as highly conductive paths for ionic transport (Lee et al., 2009; Huang et al., 2006)[1,2]. However, no study has been published that directly tests the impact of the surface and interface on the total ionic conductivity of single crystal thin film GDC. In this study, single crystal GDC films have been grown to thicknesses varying from 20 to 500 nm and their conductivities have been measured in the 500-700 degrees C temperature range. Decreasing conductivity with decreasing film thickness was observed. Analysis of the conductivity data is consistent with the presence of an approximately 50 nm layer of less conductive material in every film. This study concludes that the surface and interface regions of thin film GDC are less conductive than the bulk single crystal regions, rather than being highly conductive paths. (C) 2011 Elsevier B.V. All rights reserved.