The structural changes of Ni species in Ni/Al2O3, Ni/SiO2 and Ni/TiO2 catalysts during the repeated cycles of the methane decomposition and,the subsequent gasification of deposited carbons with CO2 have been studied. The catalytic activity for the methane decomposition depended on the size of Ni metal particles; i.e., the particle size from 60 to 100 nm was most effective. The catalytically active Ni species in Ni/SiO2 were present as Ni metal particles, with diameter from 40 to 100 nm at the first reaction cycle, but they were aggregated into ones with diameters larger than 200 nm resulting in the deactivation of Ni/SiO2 for methane decomposition. In contrast, catalytic activity of Ni/TiO2 was. kept high during the repeated reactions because the particle size of Ni metal was kept at the optimum, size range effective for the methane decomposition. On the,other hand, Ni species in a fresh Ni/Al2O3 were composed of highly dispersed Ni(II) species and Ni metal particles of a diameter smaller, than 20 nm; During the repeated reactions, the Ni(II) species were gradually reduced, forming the fine Ni metal particles, and the fine metal particles were aggregated into ones with a larger. diameter, which brought about an increase in the catalytic activity of Ni/Al2O3. (C) 2003 Elsevier Inc. All rights reserved.