Structures of Ni catalysts supported on filamentous carbon (CFC) produced by methane decomposition over coprecipitated Ni and Ni-Cu/alumina catalysts were studied by EXAFS and TEM. Thermal pre-treatment in N-2 at 350 degrees C of samples impregnated by nickel nitrate precursor was found to produce either NiO or nickel carbide, Ni3C, phase. This was explained by different reducing properties of the carbon nanofibers which depend on the surface structure. High stability of the Ni/C catalysts in methane decomposition reaction at 550 degrees C was found with those prepared from only nickel chloride precursor, due to the formation of large (30-70 nm) Ni particles further leading to new carbon filaments growth. Data implies a common mechanism of the filamentous carbon deposition in all Ni-based catalysts, independent of the support (silica, alumina, carbon) being used. However, accumulation of filamentous carbon is strongly influenced by morphology and texture of the support.