The results of a series of measurements are reported on the hydrogenation activity of the catalyst system: palladium-gold. Two types of catalysts are employed, one in the form of metal microspheres containing different compositions of the two metals as alloys, the other in the form of the mixed metals dispersed on an alphaalumina support. The alloy-microsphere system exhibits a pronounced increase in specific activity for cyclopentane hydrogenation at 500 °K resulting from the addition of gold to palladium with a maximum near 50 at.% gold. On the other hand the supported mixed-metal system exhibits an entirely different pattern in specific activity for benzene hydrogenation at 473 °K. It is concluded that although X-ray data and electron microprobe analysis indicate alloy formation for the supported catalyst, the surface phase of the crystallites is not in composition equilibrium with the bulk in spite of the fact that the Pd-Au system exhibits complete miscibility over the entire range of compositions.