Methane decomposition into H-2 and carbon nanofibers at 823 K and subsequent gasification of the carbon nanofibers with CO2 into CO at 923 K were performed over supported Ni catalysts (Ni/SiO2, Ni/TiO2 and Ni/Al2O3). Supported Ni catalysts were deactivated for CH4 decomposition with time on stream due to deposition of a large amount of carbon nanofibers. Subsequent contact Of CO2 with carbon nanofibers on the deactivated catalysts resulted in the formation of CO with a conversion of the carbons higher than 95%. In addition, gasification with CO2 regenerated the activity of supported Ni catalysts for CH4 decomposition, indicating that H-2 formation through CH4 decomposition and CO formation through gasification with CO2 could be carried out repeatedly. Conversions of carbon nanofibers into CO were kept higher than 95% in the repeated gasification over all the catalysts, while change in the catalytic activity for CH4 decomposition with the repeated cycles depended on the kind of catalytic supports. Catalytic activity of Ni/SiO2 for CH4 decomposition was high at early cycles, however, the activity decreased gradually with the repeated cycles. On the other hand, Ni/TiO2 and Ni/Al2O3 showed high activity for CH4 decomposition and the activity was kept high during the repeated cycles. These changes of catalytic activities for CH4 decomposition could be explained by changes in particle sizes of Ni metal, i.e. Ni metal particles in Ni/SiO2 aggregated into ones larger than 150 nm with the repeated cycles, while the particle sizes of Ni metal in Ni/TiO2 and Ni/Al2O3 remained at an effective range for CH4 decomposition (60-100 nm). (C) 2004 Elsevier Ltd. All rights reserved.