The room temperature (RT) formation of the interface between metals (Cr, Sn, Ag) and the SnO2 surface was studied by Synchrotron Radiation induced UPS. The Sn4d core level and the Valence Band were monitored as a function of metal growth starting from submonolayer regime. At low monolayer coverages a redox reaction with different rate and evolution depending on metal reactivity, arises between the Cr and Sn atoms and the SnO2 surface, oxidizing the overlayer and reducing the Sn cation at the interface. Deposition of less reactive Ag film leaves the stoichiometry of the substrate unchanged. However, in this case as well as for Cr, at high metal coverage the metallic tin segregation is observed.