Active and stable coprecipitated Ni-Cu-alumina catalysts were employed for direct decomposition of undiluted methane to produce CO.-free hydrogen and nanocarbon at 773-1013 K in a tubular reactor. Methane conversion increased with the reaction temperature, however, the catalyst stability declined. The weight ratio of formed solid nanocarbon and the catalyst employed attained about 380 and 280 g C/g cat at 873 and 973 K, respectively. Simultaneously, 30 and 70 vol. % hydrogen could be produced steadily for approximately 70 and 20 h, respectively. The doping of copper enhanced the catalyst stability when employed in a suitable amount. However, it made the catalyst particles to become quasi-liquid state at 973-1013 K in methane decomposition. Consequently, the catalyst particles were easily cut into small particles and encapsulated by the growing carbon layers. The tendency became pronounced with the increase of copper content. This effect may play an important role for the deactivation at high temperature in methane decomposition. (C) 2004 Elsevier B.V. All rights reserved.