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Scanning electron microscopic techniques were employed to study the surface morphological changes of oxide cathodes and nickel caps as a result of cathode activation extending over periods of 1–12 h. Elemental analysis of barium, strontium, tungsten, magnesium and aluminium was performed using energy dispersion x-ray spectroscopy. An abrupt change was observed after activation longer than 3 h. Conduction through well activated cathode assemblies was found to be due to intergranular electron tunnelling at low temperatures (T ⩽ 500 K), while trapping and detrapping at grain boundaries becomes the dominant mechanism at high temperatures (T ⩾ 500 K). The contribution of the interfacial layer to conductivity was found to be significant for cathodes activated for smaller periods.