The surface composition of a Pd-Au alloy was determined using low energy ion scattering spectroscopy (LEISS). A stable surface composition was found between 700 and 1000 K with substantial enrichment in Au compared to the bulk. Infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) were used to investigate the surface adsorption sites and ensembles. The isolated Pd sites, PdAu6 for Pd-Au on Mo(110) and for Pd/Au(111), and PdAu4 for Pd/Au(100), were observed by controlling the Pd amount and the annealing temperature. Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of Au in a Pd-Au alloy catalyst. The enhanced rates of VA formation for low I'd coverages relative to high I'd coverages on An single crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two, non-contiguous, suitably spaced I'd monomers. The results show that the role of Au is to isolate single I'd sites that facilitate the coupling of critical surface species to product while inhibiting the formation of undesirable reaction by-products. (C) 2006 Elsevier B.V. All rights reserved.