1. The reversible adsorption of benzene vapor on an alloy of Pd with 6% of Ru at 303°K under the conditions of existence of the α phase of the alloy-hydrogen system does not change, and in the region of existence of the β phase, it increases somewhat with a rise in the pressure of hydrogen. Strongly adsorbed benzene occupies∼7% of the surface. 2. Only the strongly adsorbed benzene is hydrogenated on the α phase of the alloy-hydrogen system. 3. The feeding of hydrogen to the hydrogenation zone by diffusion through the alloy leads to a supersaturation of the surface with a form of chemisorbed hydrogen that is active in the benzene hydrogenation reaction. © 1983 Plenum Publishing Corporation.