The behaviour of dense nickel substrates coated by electrochemically deposited Co3O4 was analysed in Li2CO3-Na2CO3 under the usual molten carbonate fuel cell conditions. The cobalt oxide deposit reacts rapidly yielding LiCoO2, detected by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Nevertheless, after I h of immersion in the carbonate melt, XPS shows that. in the outermost layers (< 10 nm), LiCoO2 is no longer detected and has probably gone from the substrate by mechanical disintegration or chemical transformation in the molten carbonate. The progressive loss of cobalt from the sample is proved by energy dispersive spectroscopy (EDS) and confirmed by all the other techniques (XRD, SEM, XPS) including the open-circuit-potential and cyclic voltammetry. However, the remaining cobalt detected by EDS, even after 48 h of immersion in the melt. could be progressively transformed into a mixed cobalt-nickel compound, which diffuses in the bulk. A significant decrease in the nickel solubility can be noted when using the coated substrate. (C) 2004 Elsevier B.V. All rights reserved.