The activity of silica-alumina catalyst in the isomerization of m-xylene was measured at the temperatures of 290°C, 320°C, 380°C, and 430°C in the liquid phase under a pressure of 300 kg/cm2. The observed activities were considerably higher than those in vapor-phase reactions, moreover even after 20 days practically no decline in the activity was observed except for the measurements at 430°C. This superior ability of the catalyst in the liquid phase can be ascribed to the cleaning effect of the liquid reactant: in the liquid-phase reaction the high-boiling by-product, which, in vapor-phase reactions, diminishes the activity because of its strong adsorptivity on the surface sites, is successively dissolved in the reactant; therefore, more surface sites are constantly available for the isomerization. This interpretation was substantiated by the following results: 1) when the reaction was operated in the liquid phase, about eighteen times as much high-boiling by-product was obtained as in the vapor phase; 2) the activity in the liquid-phase reaction was much higher than that in the vapor phase; 3) the decreased activity caused by the interruption of the flow was regenerable when the flow was resumed; 4) the addition of the high-boiling by-product to the reactant m-xylene resulted in a considerable decrease in the activity. In addition, the identification of the components in the high-boiling by-products was carried out by means of mass and NMR spectroscopic studies.