Bimetallic catalysts exhibit markedly different properties from either of the constituent metals, and enhanced catalytic performance. Surface composition is a key to understanding the role of alloying in mixed-metal catalysts. Scanning tunneling microscopy (STM), low-energy ion scattering spectroscopy (LEIS), infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) using CO as a probe molecule were used to elucidate the surface concentration and adsorption sites as well as the surface ensembles. PdAu(100), Pd/Au-low index single crystal surfaces, and planar model surface of Pd-Au/Mo(110) were selected as model surfaces. Isolated Pd sites, PdAu6 for Pd-Au on Mo(110) and for Pd/Au(111), and PdAu4 for PdAu(100) and 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 Pd coverages relative to high Pd coverages on Au single crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two, non-contiguous, suitably spaced Pd monomers. The results show that the role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product while inhibiting the formation of undesirable reaction by-products.