The reaction of sulfur with Au/Pt(lll) and Ag/Pt(lll) surfaces has been investigated using high-resolution photoemission spectroscopy with synchrotron radiation. The valence spectra of S/Au/Pt(lll) and S/Ag/Pt(111) surfaces display weak signals at binding energies between 1.8 and 0 eV. The adsorption of S induces a large decrease in the density of states displayed by the bimetallic surfaces around the Fermi level. This phenomenon may be responsible for the decrease in chemical and catalytic activity that accompanies sulfur poisoning. In S/Au/Pt(lll) surfaces, sulfur does not form compounds with Au or Pt. Au forms three-dimensional clusters, while S segregates into two-dimensional islands of high local coverage where it is bonded to hollow and bridge sites of the Pt substrate. In S/Ag/Pt(111) surfaces, silver reacts with sulfur and promotes the synthesis of platinum sulfides. The formation of silver and platinum sulfides makes possible the adsorption of large amounts of sulfur on Ag/Pt(111) surfaces. The S/Au/Pt(111) and S/Ag/Pt(lll) systems illustrate different extremes of the phenomena that can occur when sulfur is present on a bimetallic surface. The differences in the behavior of the S/Au/Pt(111) and S/Ag/Pt(111) surfaces arise from variations in the nature of the admetal <----> S and admetal <----> Pt interactions.