Bimetallic Pt-Au/SiO2 catalysts were prepared by reduction of AuCl4- using hydrogen preadsorbed on a parent monometallic Pt/SiO2 catalyst. In fact during that preparation a direct redox reaction can occur between the reduced platinum and AuCl4-. At low gold coverage the additive is deposited statistically on the whole surface of platinum particles (the reducer is H preadsorbed on Pt). At high gold coverage the additive is preferentially deposited on the high index planes of platinum particles (by direct redox reaction). During the formation of Pt-Au bimetallic entities sintering of platinum can occur as a result of simultaneous oxidation and reduction of platinum.