The equilibrium dissociative adsorption of CH4 was studied over Ni-YSZ cermet catalysts for a deeper insight regarding the nature and binding strength of generated carbon species on the Ni-YSZ surface. Three main carbon species were detected by the reaction of carbon adspecies with H-2 to produce CH4 or with O-2 to produce CO and CO2. Carbidic species (C-c) are reactive with H-2 and O-2 at temperatures below 600 K while adsorbed carbon (C-a) species in equilibrium with CHx, species react with H-2 and O-2 above 600 K. Graphitic carbon layers (C-g) are formed upon CH4 adsorption above 700 K and its main characteristic is the absence of any reactivity with H-2, to form CH4. The binding energy of C-a species with respect to graphite decreases with increasing coverage ranging between 7.32 +/- 0.03 and 6.5 +/- 0.04 eV for the low (< 0.2 ML) and high (approximate to 1 ML) coverage, respectively. The presence of 1% wt of Mo either suppresses the formation of adsorbed graphitic layers which are not reactive with H-2 or enhances the reactivity of adsorbed hydrogen atoms toward CH4 at temperatures higher than 800 K, thus revealing the positive effect of Mo in inhibiting the formation of adsorbed graphitic layers. (C) 2003 Elsevier Science (USA). All rights reserved.