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At the last international vacuum electron sources conference, held in Saratov, Russia, the current authors presented a paper reporting on preliminary findings from an investigation into the interfacial layer that forms in the oxide cathode over accelerated cathode life (Jenkins et al., applied surface science, 215, 78. 2003). It was reported that studies were being conducted to investigate key issues that could bring about improvement in performance to the oxide cathode, in order for the oxide cathode to remain a competitive display technology. One of the key issues identified was the need for characterisation of the interface layer that forms during the operation of an oxide cathode. Although this subject has been investigated extensively before (Rooksby 1940) (Rooksby 1947) (Eisenstein 1949) (Wright 1947), ambiguity remains concerning both the nature and the effect of the interfacial layer on the performance of the oxide cathode. There is firm evidence to indicate that this interfacial layer blocks the 'activating' elements which are necessary for the reduction of the barium oxide in the emissive layer to give the free barium that lowers the work function. This blocking effect, therefore, reduces the performance of the cathode. In this study, the interfacial layer was characterised using a number of analytical techniques in order to obtain a comprehensive understanding of the geometry and chemistry of this layer. The techniques that have been used are scanning electron microscopy (SEM), Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM).