A multi-technique approach has been used to investigate coke formation and the effects of coke formation during n-hexane cracking on ultrastable zeolite Y. The product analysis reveals that propane and propene are the major products, and this suggests that the reaction initiation step may be direct protonation at very strong Bronsted acid sites. The coke formed does not change in composition significantly over the course of the reaction. Infrared spectroscopy reveals that the vast majority of Bronsted and Lewis acid sites are still present in the catalyst, even when the catalyst activity has become low. Diffusion measurements using pulsed field gradient (PFG) NMR show that the mobility of adsorbed n-butane or n-hexane is not affected by the presence of the coke, and so shows that pore blockage is not a significant factor. Based on the experimental results, it is argued that selective site poisoning of a few very strong acid sites is the main deactivation mechanism for this reaction under the conditions employed.