Ru Pd/SiO 2 catalysts were characterized by hydrogen chemisorption, XPS and TPD of CO, and their catalytic activity was obtained for the gas phase hydrogenation of o-xylene at atmospheric pressure. A small maximum in activity appeared in the bimetallic catalysts at temperatures at which palladium is inactive. The stereoselectivity of the bimetallic catalysts differed markedly from that of monometallic ruthenium. A ligand effect between surface Ru and Pd atoms is suggested to be responsible for the catalytic behaviour of the bimetallic catalysts.