Ethylene decomposition over Ni supported on novel carbon nanotubes (CNT) and nanofibers under consecutive reaction/regeneration cycles to form CO-free hydrogen and carbon deposits have been investigated. The present work highlights the effects of support chemical composition, catalyst synthesis method and Ni metal loading on the catalytic activity and stability of nickel. A novel 0.5 wt.% Ni/CNT catalyst which presents the highest value of a constant hydrogen product yield (17.5 mol H-2/mol Ni), following consecutive reaction (complete deactivation) -> regeneration (20% O-2/He, 400 degrees C) cycles, ever reported in the open literature has been developed. Transmission electron microscopy (TEM) and XRD studies revealed that the 0.5 wt.% Ni/CNT catalyst promotes the formation of two types of carbon nanofibers during ethylene decomposition at 400 degrees C, result that reduced significantly the rate of Ni encapsulation during reaction. The opposite is true in the case of 0.3 wt.% Ni/SiO2 catalyst. In the case of Ni/CNT, there is a narrow range of Ni loading for which maximum H-2 product yield is obtained, while in the case of Ni/SiO2, a monotonic increase in the H2 product yield is obtained (Ni loading in the range 0.3-7.0 wt.%). (c) 2005 Elsevier B.V. All rights reserved.