Silica-supported bimetallic Pt-Au catalysts prepared via different synthetic routes have been investigated in terms of their structural properties, adsorption of CO, and catalytic activity for the selective catalytic reduction of NO by propylene, the oxidation of propylene in the absence of NO, and the O-16/O-18 homoexchange reaction. Catalysts prepared by incipient wetness impregnation from individual Pt and An precursors exhibited characteristics very similar to those of monometallic Pt catalysts, indicating that in these cases the presence of Au did not affect the catalytic performance of Pt in any significant way. This behavior is consistent with a model in which the two metals remain segregated due to their miscibility gap, and only Pt participates in the adsorption of CO and the reactions under consideration. In contrast, catalysts prepared from a Pt2Au4(C equivalent to (CBu)-Bu-t)(8) organo-bimetallic cluster precursor exhibited different behavior both in terms of CO adsorption and their catalytic activity for the three reactions examined. The combination of the kinetic, spectroscopic, and structural characterization data suggests that in this case Pt and An remain intimately mixed in the form of Pt-Au bimetallic particles and that the presence of Au in these particles modifies the behavior of Pt.