The catalytic activity of o-xylene transformation and cyclopropane isomerization over hydrogen reduced AgY was greatly enhanced by the presence of gaseous hydrogen. This enhancing effect was reversible by the presence and absence of gaseous hydrogen, and was confirmed by the catalytic activity changes and infrared spectroscopy. While, HY had not enhancing effect in the presence of hydrogen. This hydrogen enhancing effect and its reversibility over reduced AgY may be related to the redox properties of the small silver clusters which can be chemisorbed gaseous hydrogen. This effect was also affected by the partial pres-sure of hydrogen, and the extent of Ag⁺ ion-exchange.