Methane decomposition into hydrogen and filamentous carbons was carried out over Fe2O3/Al2O3 and Fe2O3/SiO2 (7-77 wt% as Fe2O3) at 1073 K. The carbon yield in the methane decomposition for Fe2O3/Al2O3 (22.5 g-C/g-Fe) was greater than that for Fe2O3/SiO2 (7.5 g-C/g-Fe). The difference of the catalytic performance between the two catalysts can be explained by the particle size of the catalytically active species (alpha-Fe metal and Fe3C); i.e., an average particle size of catalytically active species in Fe2O3/Al2O3 did not change appreciably despite an increase of loadings, while the size in Fe2O3/SiO2 became larger with loadings. Fe2O3 crystallites of diameters smaller than ca. 30 nm in the fresh catalysts were transformed into a-Fe metal and Fe3C (cementite) immediately after contact of methane at 1073 K, while those of larger diameters were transformed into gamma-Fe metal saturated with carbon atoms (austenite). The structures of carbons formed by methane decomposition were dependent on the types of catalytic support. Fe2O3/Al2O3 catalysts formed multiwalled carbon nanotubes and chain-like carbon nanotubes. On the other hand, Fe2O3/SiO2 catalysts formed filamentous carbons which were composed of many spherical carbon particles without a hollow structure, in addition to chain-like carbon nanotubes. (C) 2003 Elsevier Inc. All rights reserved.