It is demonstrated that ultrathin and thin films of Sm on Si (100) strongly promote the oxidation of Si. Photoemission measurements of the Si 2p core electrons show binding-energy shifts characteristic of Si2O3 and SiO2. The oxygen binding is conditioned by the presence of Sm, which is also oxidized. Reflection electron-energy-loss spectra and valence-band photoemission data indicate formation of an insulator with a valence-band maximum 4 eV below the Fermi level. The influences of samarium coverages, in the 1–20-monolayer regime, and of the temperature on the reaction are studied.