Impedance spectroscopy has been used to separate grain interior and grain boundary conductivities in a series of measurements on thin Ce0.8Gd0.2O2-delta ceramic electrolytes with a range of different grain size distributions. The "brick layer" microstructural model has been used to provide an estimate of a true grain boundary conductivity and to relate the electrical properties of the ceramic to microstructural parameters. High resolution electron microscopy images show that insulating intergranular phases do not necessarily have to be present in order to cause a large grain boundary resistance, A clear relationship between grain boundary resistance and the number of grain boundaries was observed for samples of mean grain size of 3 mu m or larger. For samples of sub-micron average grain sizes, measured grain boundary conductivities were inconsistent with model predictions, being significantly higher than predicted.