The interaction between a(root3 x root3)R30 degrees -Au/Si surface with thin (less than or equal to2 ML) Ni films with different dimensions deposited at 300 K is studied by means of a scanning photoelectron spectromicroscopy. The interfacial processes occurring at different Ni coverages and annealing temperatures are probed with lateral resolution of 0.12 mum. The results have revealed that Ni displaces Au from the Si surface and the presence of Au lowers the formation temperature of the Ni disilicide phase. It has been found that the behavior of the Ni/Au-Si interfaces at high temperatures, in particular the changes in the morphology of the interface and the formation of well defined Ni-x(Au1-x)Si-2 islands, is strongly influenced by the dimensions of the Ni patches. The observed evolution of the Ni/Au-Si interfaces is interpreted considering the high solubility of Ni in Si, the miscibility between Ni and Au and the Au-Si alloying at elevated temperatures.