Conductive mode (CM) and cathodoluminescence (CL) microscopy techniques were used to study grain boundary structures in a zinc oxide based varistor, doped with 0.5 mol % Bi2O3 and 0.5 mol % Sb2O3. By combining these two techniques specific details of the electrical and luminescence properties of individual interfaces could be characterised. CM imaging clearly showed the presence of potential barriers at the grain boundaries. The same grain boundaries were regions of strong CL emission. It is suggested that the dominant CL emission at grain boundaries in this material originates from self-excitation centres at impurities and/or defects within the structure rather than the direct recombination of electron-hole pairs across the forbidden band gap.