The metal-support interaction between rhodium and silica has been studied by x-ray photoelectron spectroscopy for a Rh/SiO2/Mo model system. This system consists of a thin silicon oxide layer, prepared by chemical vapor deposition on molybdenum with a nominal load of one monolayer rhodium, Heating in ultrahigh vacuum (UHV) results in changes of the cluster size and binding energies of surface species. Thermal treatments above 850 K in UHV results in the formation of a rhodium silicide, Rh3Si, which has not been reported so far. For the formation of this new phase a surface reaction mechanism is proposed.