The progressive deactivation of various HNaY zeolite catalysts (Si/Al=2.4), during methylnaphthalene isomerisation, was studied at 573-663 K. The activity decay was expressed as a function of time-on-stream (t-o-s) and of coke content (C-c). In both cases very simple exponential equations, whose parameters depend on the degree of decationation of the zeolite and temperature were satisfactorily used to fit the experimental data. Coke analysis was performed on the most acidic catalyst (HNaY-98) at T=633 K and the composition of the coke soluble in methylene chloride was determined. The influence of temperature and ion-exchange degree on the coke deposition rate was also examined.