Alloyed catalysts receive considerable attention, because of their unique catalytic properties; they often show higher selectivity, activity, and stability compared to the pure metal particles. To provide insights in the origins of these features, we report the structure and the interaction of hydrogen with each of the metals in an intimately mixed platinum-gold catalyst and compare these characteristics to those in the single metal particles. X-Ray absorption spectroscopy (XAS) and electron microscopy analysis showed that the structure of the mixed particle differed from the single metal particles. The interaction of platinum with hydrogen is stronger than the H-Au interaction and the adsorption sites were different. EXAFS analysis showed that the structure of the platinum clusters changes with increasing hydrogen coverage, observed as a relaxation of the contracted Pt-Pt distance and an increase in the Pt-Pt coordination number. No such changes were observed for gold clusters. Well-mixed PtAu-alloy clusters, with a bulk Au-to-Pt ratio of two, supported on SiO(2), adsorb hydrogen on both platinum and gold atoms, which indicates that gold cannot be regarded as an inert metal. The heat of adsorption on the platinum ensembles does not decrease upon alloying; the weakening of the overall hydrogen adsorption strength when alloying platinum with gold is an ensemble-size effect.