For the co-adsorption of An and Cu on Ru(001), the Ru substrate promotes the alloying of the noble metals by lowering the activation energy required for the process. High-resolution photoemission spectroscopy with synchrotron radiation and X-ray photoelectron spectroscopy have been used to investigate the core-level binding energy shifts and the valence-band behaviour in a series of Au and Cu overlayers on Ru(001) : 1 ML Au, 1 ML Cu, 1 ML Au and 1 ML Cu, 3 ML Au and 1 ML Cu, as well as 1 ML Au and 3 ML Cu. A monolayer of Au in contact with Ru(001) has a Au 4f(7/2) binding energy similar to 0.23 eV higher than that of Au surface atoms on pure Au but still similar to 0.1 eV smaller than that of the bulk Au. In contrast a monolayer of Cu bonded to Ru(001) exhibits a Cu 2p(7/2) binding energy similar to 0.1 eV lower than that of a Cu surface atom on pure Cu. Most of the co-deposited Au-Cu systems formed ultrathin alloys on top of the Ru(001) surface. The strain and electronic perturbations induced by the Ru substrate on the Au and Cu overlayers favour the formation of Au-Cu alloys, Binding energy shifts of the Au 5d band components and the accompanying change to the overall d-band width were found to be very sensitive to the alloying process, as has been observed in bulk Au-Cu alloys, The order of deposition, annealing, and the temperature of the substrate were all found to have an effect on the formation of Au-Cu ultrathin alloys. These ultrathin alloys exhibit Au 4f(7/2) and Cu 2p(3/2) binding energies that are significantly different from those of bulk Au-Cu alloys as a consequence of the interactions with the Ru(001) substrate.