The interfacial reactivity of the rare earth metals Yb and Pr deposited on ultra-thin layers of SiO₂ on Si(111) substrates under UHV conditions is investigated using the Auger spectra of silicon, oxygen and the rare earth metals to monitor chemical changes. At room temperature exposure to approximately 10 AA of rare earth metal leads to reduction of the SiO₂ to form rare earth oxide and rare earth silicide, while at elevated temperature reaction to form a silicate phase is demonstrated.