The interface properties of thin Gd2O3 films grown on Ge(001) are studied as a function of the oxidizing species used during the deposition. The mediation of molecular oxygen during growth produces a crystalline oxide with an atomically sharp and contamination-free interface on the Ge substrate. Conversely, an interface layer of substoichiometric germanium oxide occurs whenever atomic oxygen radicals are used. The two different Gd2O3∕Ge interfacial configurations are discussed basing on thermodynamic arguments.