Thermo Catalytic Decomposition of methane to produce CO2-free hydrogen has been carried out in a fixed bed reactor at different operating conditions using a commercial Ni-based catalyst to study the characteristics of the carbon produced and the catalyst deactivation mechanism. At temperature of 700 degrees C, the concentration of hydrogen was around 80%, which corresponds to a methane conversion close to the thermodynamic values. It has been shown that time for catalyst deactivation depends on the operating conditions, so that, the higher the temperature and methane flow, the shorter the life of the catalyst. At temperature of 700 degrees C and space time of 1s, the catalyst activity did not decay after 8 h in stream. In contrast when space time was reduced to 0.2 s, the catalyst became deactivated after 90 min while the quantity of carbon deposited was the half. Scanning electron microscope and transmission electron microscope examination has shown that the deposited carbon appears either as large filaments a few nanometres in diameter emerging from Ni particles or as uniform coatings. X-ray diffraction, Fourier transform-Raman and X-ray photoelectron spectra reveal that in both cases the carbon deposited is highly ordered graphite whose structure does not depend on the operating conditions. Formation of carbon filaments, which is desirable, is favoured by operating conditions promoting low rates of methane conversion. On the contrary, operating conditions promoting high decomposition rates enhance coating carbon deposition and shorten the catalyst life. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.