Ultrathin Y2O3 films were electron beam evaporated in an ultrahigh vacuum onto Si(100) and investigated by high-resolution medium energy ion scattering. Selected films were capped in situ with amorphous Si. Uncapped films that were exposed to air prior to analysis contained excess oxygen compared to a stoichiometric Y2O3 film, and showed a 6–8 Å interfacial layer. Si uptake from the substrate occurred in these films after a 700 °C vacuum anneal, presumably by reacting with the excess oxygen. Si-capped Y2O3 films on the other hand were stoichiometric, and the substrate interface was sharp (≤2 Å), even after 900 °C vacuum anneals. No change was seen at the Y2O3 capping layer interface until ≥800 °C for vacuum anneals. These measurements indicate that control of the interface composition is not possible after exposure of ultrathin Y2O3 films to air. © 2001 American Institute of Physics.