A steady-state simulation model for xylene isomerization over H-Y-Zeolite was developed. Liquid and vapour phase reactions were considered. The fixed bed catalytic reactor was simulated as a plug flow reactor that operates either isothermally or adiabatically. Four temperature levels were investigated taking into consideration catalyst deactivation. The main reactions considered in the model were the isomerization reactions and the disproportionation of xylenes to toluene and trimethylbenzene. The model predicts the concentration and temperature profiles for a given input of operating conditions. The simulation results revealed the similarity in the reactor behaviour if it operates either isothermally or adiabatically with respect to the distribution of the reaction products and catalyst deactivation. The study also indicated that performing the reaction in the liquid phase is more favourable than in the vapour phase due to better selectivity, relative productivity, efficiency and approach to equilibrium.