Physico-chemical properties and catalytic activity of silver exchanged L-type zeolite were studied by infrared spectroscopy of adsorbed CO and pyridine, X-ray diffactometry, TPR(temperature proprammed reduction), hydrogen adsorption and acid-catalyzed reactions. Hydrogen was chemisorbed reversibly on the reduced silver exchanged L-type zeolite, and the active site for the hydrogen adsorption was related to a silver cluster such as Ag_n⁺. For o-xylene transformation, the catalytic activity of AgL-48 in the presence of hydrogen was much higher than that of nitrogen alone as a carrier gas. This enhancing effect of hydrogen was reversible by the presence and the absence of gaseous hydrogen in the reaction system. This enhancing effect was also observed in the other acid-catalyzed reactions, and the chemisorbed hydrogen on the reduced AgL-48 zeolite was retained as acidic protons in the zeolite.