Preliminary investigations are presented into the grain boundary structure of gadolinia doped ceria by high-resolution transmission electron microscopy (HR-TEM) imaging using a 300 kV field emission FEI TITAN 80/300 monochromated transmission electron microscope, with a spatial resolution of <0.2 nm. Impedance spectroscopy over the temperature range 150-350 degrees C and frequency range 0.1 Hz-10 MHz has been used to separate the contribution to the total conductivity of the grain interior and grain boundary for ceramic samples. The samples were fabricated from powder of composition Ce1-xGdxO2-delta, x=0.1 (CGO10) and x=0.2 (CGO20) which were obtained from three different commercial suppliers. These impedance measurements have been correlated with scanning electron microscopy (SEM) images of the material microstructure in order to estimate the specific grain boundary conductivity. The bricklayer model has been used to interpret the impedance data, which show wide variation in specific grain boundary conductivity between samples of the same nominal composition. No grain boundary second phases have been observed at any boundaries and the grain boundary blocking effect has been attributed to the presence of space charge layers. HR-TEM studies at these interfaces reveal defect structures, which may play a significant role in the grain boundary conductivity behaviour of these materials. (C) 2008 Elsevier B.V. All rights reserved.