We used X-ray photoelectron spectroscopy and temperature-programmed desorption (TPD) to investigate the oxidation of Tb 2 O 3 (111) films on Pt(111) by gaseous oxygen atoms. We find that plasma-generated O atom beams are highly effective at completely oxidizing the Tb 2 O 3 films to TbO 2 at 300 K, for film thicknesses up to at least seven layers. Heating to ∼1000 K in ultrahigh vacuum restores the films to the Tb 2 O 3 stoichiometry and produces two distinct O 2 TPD features centered at ∼385 and 660 K, which we attribute to the release of lattice oxygen from the surface vs bulk trilayers, respectively. We also find that the adsorption of plasma-activated oxygen at 90 K produces a weakly bound state of oxygen on the TbO x films which desorbs between ∼100 and 270 K during TPD. This oxygen state is consistent with a form of chemisorbed oxygen, possibly an atomic and/or molecular species that bonds on-top of Tb atoms at the surface. TPD experiments of the oxidation of Tb 2 18 O 3 films by 16 O atom beams demonstrate that oxygen desorption below about 500 K originates almost entirely from the oxygen that is "added" to the Tb 2 O 3 film and that all isotopic combinations of O 2 desorb from the bulk above 500 K, though the relative amount of 18 O to 16 O which desorbs above 500 K is lower than that determined from the isotopic composition of the oxidized TbO x films. These results support the idea that oxygen desorption below 500 K originates from oxygen species that are localized at the surface and further suggest that the oxide structure only partially accommodates oxygen atoms that incorporate into lattice sites at 300 K.