Reactions of (CH4)-C-13 and (CH4)-C-12 in the presence of either CO2 or CO over reduced and oxidized Ni-MgO catalyst were studied to obtain mechanistic information on carbon deposition at low and high pressure. As the reaction time increased the amount of carbon deposited on the catalyst also increased. The carbon deposited on the catalyst during the first hour of reaction appears to be very active and was removed readily by reacting with 2 vol.% oxygen in helium at 660 degreesC and the catalyst recovered its original activity and selectivity after removal of carbon. During the temperature-programmed oxidation (TPO) of deposited carbon the catalyst Consumed a significant amount of oxygen forming nickel oxide, only a fraction of which is consumed by reacting with carbon species on the surface. This indicates that the surface oxygen plays a significant role in oxidizing and removing carbon species from the catalyst surfaces. Pulse reaction and oxidation experiments show that methane is activated on the surfaces of both the oxidized and reduced catalyst whereas the CO2 activation requires the catalyst to be in reduced state. Carbon deposited on the catalyst is formed from both methane and CO2, possibly via disproportionation of CO which is produced from both (CH4)-C-13 and (CO2)-C-12. (C) 2004 Elsevier B.V. All rights reserved.