In this paper we demonstrate the use of variable temperature deuterium line shape and relaxation time studies to characterise the modification to adsorption sites and thermally activated inter-site jump processes occurring within zeolite HY at various stages during coke deposition. In the examples presented here the adsorption and transport properties of probe benzene molecules are studied. Coke was produced using two different feed molecules; ethanol and n-hexane. The data presented show that the extent to which the sites for benzene adsorption are modified during coking depends both on the time-on-stream and the nature of the feed molecule producing the coke. The results suggest that coke produced by ethanol deactivates the zeolite by selective coverage of one of the two types of site initially available for benzene adsorption. After 15 h on stream there is evidence that the effective co-ordination of the pore network has been reduced from 4 to 3.4. In contrast, deactivation by n-hexane cracking causes deposition of coke within the pore structure which blocks adsorption of probe molecules onto the native adsorption sites of the zeolite.