The LaCoO3 perovskite-type catalysts in this study were monoliths resting on supports made of heat-resisting foil and washcoated with Al2O3. The La0.9Ag0.1CoO3 or La0.92Pd0.08CoO3 perovskite was used as the active phase. Partial substitution of lanthanum in the LaCoO3 perovskite with palladium or silver enhances the activity of the monolithic catalysts in the combustion of methane. XPS and XRD analyses show that during approximately 500 h on stream (at 750 and 704 degreesC, respectively) the surface composition of the La0.9Ag0.1CoO3 catalyst and that of the La0.92Pd0.08CoO3 catalyst undergoes considerable changes. In both the catalysts, cobalt and aluminum oxide segregate to the surface while the amount of carbonate groups decreases. In the La0.92Pd0.08CoO3 catalyst, the amount of palladium and the average oxidation state of Pd on the surface increase. On the surface of the La0.9Ag0.1CoO3 catalyst, the amount of silver decreases, and Ag in the metallic state oxidizes to Ag+, which probably becomes built-in the perovskite structure. In spite of these changes, the catalysts display a high activity and a good stability during above 500 h of methane combustion. (C) 2004 Elsevier B.V. All rights reserved.