Thin Lu2O3 and Yb2O3 layers grown on Si using Atomic Layer Deposition (ALD) were studied with x-ray photoelectron spectroscopy (XPS) and low energy ion spectroscopy (LEIS). Our results show that ALD grown Lu2O3 and Yb2O3 films subsequently annealed in N2 are polycrystalline, and that their oxygen “subsystems” are very sensitive to further heat treatments in various atmospheres. In particular, it is evident that two chemically nonequivalent oxygen states are present in the oxides at equilibrium conditions. The one with the higher binding energy (BEO1s ≈ 531.8 eV) is loosely bound, desorbs from the subsurface region upon vacuum annealing at T>300°C, and reappears upon annealing in O2 (T≈500°C, PO2=10-1 Torr). ALD can provide a variety of Lu silicates, depending on precursor combination and growth conditions. As grown ultrathin (≤5 nm) continuous Lu2O3 and …