The coadsorption of Au and S on Mo(110) and Rh(111) has been examined using thermal desorption mass spectroscopy, photoemission, and ab initio self-consistent-field calculations. On both surfaces, the interactions between Au and S are repulsive. S reduces the adsorption energy of Au. In some cases, the weakening of the Mo-An and Rh-Au bonds is so large (5-7 kcal/mol) that the Au adatoms "ball up" on the surface instead of "wetting" the Mo(110) and Rh(111) substrates. The ab initio SCF calculations show that a S adatom modifies the chemical behavior of adjacent Mo and Rh sites by reducing their contribution to the density of states around the Fermi level of the system, weakening in this way their ability to form strong bonds with Au. In the Au/S/Mo(110) and Au/S/Rh(111) surfaces, Au and S compete for the electrons of the transition-metal substrate. Gold produces dramatic changes in the reaction pathways for the removal of sulfur from the Mo and Rh surfaces. In the Au/S/Rh(111) system, S-2 desorbs at much lower temperatures (100-250 K) than in S/Rh(111). For Au/S/Mo(110), the presence of Au favors desorption of St instead of migration of S into the bulk of Mo. These results are consistent with a model in which An compresses the S overlayer into islands of high local coverage, reducing in this way its stability and favoring the "pairing" of sulfur atoms and desorption of S-2.