Interfacial interactions between the nickel cap and emissive oxide in double carbonate thermionic oxide cathodes have been studied as a function of operational lifetime by SEM, AES, EDX and TEM. The surface of the interface region has been studied by stripping away the oxide from the cap. In cross-section, the interface has been examined by (a) preparing 10 degrees taper crosssections and (b) cutting FIB sections. Cracks are observed to develop in the bulk Ni and become more pronounced as a function of operational lifetime. Cracks are found both at grain boundaries and close to the surface, parallel to the interface. It is proposed that the cracks develop from voids, formed as a result of the high diffusivity of Mg and Al in the Ni matrix. EDX shows the presence of Al, Mg, Ba, Sr and O within the cracks and it is proposed that M-O and (Ba, Sr)Al2O4 are two major reaction products. A thin (skin) layer containing Al, Ba, Sr and O forms at the Ni cap/emissive layer interface, attributed primarily to (Ba, Sr)Al2O4 formation. (c) 2005 Elsevier B.V. All rights reserved.