A study of the kinetics for the liquid phase isomerization of ortho-xylene over H-mordenite has been made at 505°K, 2.76 × 105 N/m2, and 23 mole % toluene diluent. This is the first known study reporting kinetic data for the liquid phase isomerization of xylene over a zeolite catalyst. High activity is indicated by the 80% achievement of para-xylene equilibrium at 505°K, 2.76 × 105 N/m2, and 0.767 × 10−3 kg/s/kg. Although high selectivity has previously been reported for a specifically prepared zeolite catalyst, the 99+% selectivity of the isomerization reaction with H-mordenite as indicated by no C9+ aromatics and less than 0.5 mole % benzene in the product is most significant. Catalyst deactivation was also significant. Reaction rate constants are reported for a first-order reversible triangular model among the xylene isomers with catalyst deactivation incorporated into the model.