The dry and oxidative dry reforming of CH4 over alumina-supported Co-Ni catalysts were investigated over 72-h longevity experiments. The deactivation behaviour at low CO2:CH4 ratio (<= 2) suggests that carbon deposition proceeds via a rapid dehydropolymerisation step resulting in the blockage of active sites and loss in CO2 consumption. In particular, at high temperatures of 923 K and 973 K, a 'breakthrough' point was observed in which deactivation that was previously slow suddenly accelerated, indicating rapid polymerisation of deposited carbon. Only with feed CO2:CH4 > 2 or with O-2 co-feeding was coke-induced deactivation eliminated. In particular, O-2 co-feeding gave improved carbon removal, product H-2:CO ratios more suitable for downstream GTL processing and stable catalytic performance. Conversion-time data were adequately fitted to the generalised Levenspiel reaction-deactivation model. Activation energy estimate (66-129 kJ mol(-1)) was dependent on the CO2:CH4 ratio but representative of other hydrocarbon reforming reactions on Ni-based catalysts. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.