Methane decomposition was carried out in the presence of CO2 over the nickel catalysts. Spherical alumina and glycothermally synthesized zirconia were used as the catalyst supports. In the presence of CO2, CH4 was decomposed in the same fashion as pure methane decomposition, and fibrous carbons were formed. However, the formation of hydrogen, carbon monoxide, and water continued even after the apparent carbon formation ceased, and this phenomenon was observed irrespective of the support materials. These results showed a sharp contrast against the results for the pure methane decomposition where the catalyst was completely deactivated when the carbon formation ceased. Further carbon formation was observed when the feed gas containing CO2 was replaced with pure CH4. Mechanisms for these phenomena are discussed front the thermodynamical point of view. (c) 2007 Elsevier Ltd. All rights reserved.