Gd oxidizes preferentially at the (0 1 0) surface of Gd₅Ge₄. This is consistent with thermodynamic data for the bulk oxides. Upon oxidation in vacuum, the gadolinium oxide displaces or covers the Ge. Oxidation is more extensive at 600 K than at 300 K, because more oxygen is incorporated into the surface and the shift of the Gd binding energy is larger.