A series of progressively proton-exchanged sodium mordenite samples was studied by operando infrared spectroscopy during xylene conversion. This unique, powerful tool allowed us to characterize the species adsorbed to the surface during catalysis and to follow the evolution of the active sites as the reaction proceeded. We were then able to correlate the surface data with the catalytic results obtained by gas-phase chromatography analysis. When very few hydroxyls are present on site O7H close to the main-channel walls, the initial isomerization selectivity is very high and the high Na+ content leads to the preferential adsorption of 1,2,4-TMB inside the micropores. When the amount of acidic hydroxyls increases, new sites O2H are generated at the intersection between main channels and side pockets. The initial disproportionation selectivity then increases roughly. Finally, the last hydroxyls appear on site O9H located at the end of the side pockets. These constrained hydroxyls are not accessible to the reactant in the reaction conditions, but their presence leads to a global activity increase without any selectivity modification. Acid strength considerations, together with the observation of the "working hydroxyls," indicate an indirect effect of O9H on the overall activity, which is probably related to a modification in the void space inside the micropores. (C) 2004 Elsevier Inc. All rights reserved.