The deposition at room temperature of Au films on Si(100) has been studied by a combination of techniques for thicknesses up to 10 monolayers. From low-energy ion scattering (LEIS) and Auger electron spectroscopy (AES) measurements, it is derived that the outermost layer consists of a mixture of Au and Si for the whole coverage range. The critical coverage to induce intermixing is determined to be at most 0.25 monolayer (ML), but the measurements are also consistent with an immediate onset of intermixing. The surface silicide has a disordered character as shown by spot-profile analysis low-energy electron diffraction (SPA-LEED). Using this technique it is also concluded that the interface between the surface silicide and the substrate is essentially identical to the morphology of the Si(100) surface before deposition.