Coking over two H-mordenites (framework SiO2/Al2O3 ratios of 15 and 75) was studied during o-xylene isomerization at temperatures between 523 and 723 K. Despite its lower acid site density the deeply dealuminated sample was the most active and more stable. This is due to the presence of mesopores created during the dealumination which render the diffusion of organic molecules quasi tridimensional, thus reducing both diffusive constraints and the deactivating effect of coke, with both samples coke aromaticity increased and the yield in coke soluble in methylene chloride decreased as the temperature increased. The composition of this coke fraction was strongly affected by reaction temperature but not by the characteristics of the mordenite samples. Whatever the temperature, coke components remained trapped in the mordenite pores, At low temperature, this trapping was due to their low volatility and/or strong adsorption, at high temperature to their steric blockage. Coke oxidation was practically independent of the coke content and composition but was favoured by the density of the acid sites.