The influence of the carrier gas (H-2 or N-2), at different pressures, on coking over two mordenite samples during o-xylene transformation was evaluated at 623 K. The results showed that the carrier gas has a significant effect on the amount and on the composition of the coke formed. For the two samples used, coke formed in the presence of H-2 consisted mainly of CH2Cl2 soluble molecules, while under N-2 the formation of insoluble coke was favoured. Whatever the carrier gas or the total pressure, the same families of compounds were identified in the soluble fraction, the main ones being CnH2n-28, CnH2n-22 and CnH2n-16. For the sample strongly dealuminated, increasing the pressure decreased the coke build up under H-2, confirming the inhibition effect of H-2 on coke formation, but an opposite effect was observed under N-2. Due to its mesoporous system, this sample has a rather stable activity whatever the carrier gas. For the sample with a high density of acid sites, the limiting quantity of coke was rapidly attained, deactivation was relatively fast and poisoning by coke was greater under N-2. A relatively small amount of coke render it inactive indicating that deactivation is due to pore mouth blockage.