We have studied the chemical and compositional modifications induced by 8 keV Ar° irradiation at the surface layers of ZrO2, SiO2, and ZrSiO4 (zircon). The surfaces were examined by in situ x-ray photoelectron spectroscopy (XPS). We have found that while pure SiO2 evolved to SiO1.95 and pure ZrO2 evolved to ZrO1.85, involving the appearance of reduced Si and Zr species in the XPS spectra. On the other hand, ZrSiO4 underwent a major loss of SiO2, to yield a final Si/Zr ratio of 0.63, and a strong degree of ZrIV reduction (to yield ZrO1.70) with respect to pure ZrO2. The experimental results are discussed in terms of several possible mechanisms, including the kinetic loss of O due to high diffusivity in the perturbed collision cascade volume, the occurrence of “bombardment-induced segregation” (BIS) phenomena, the occurrence of redox reactions, as well as the “lattice relaxation with chemical guidance,” related to the thermodynamic stability of irradiation-induced compounds. In particular, the compositional modifications for simple oxides seem to be well explained mainly in terms of the model of lattice relaxation, i.e., in terms of the “thermodynamically allowed” formation of the intermediate valence species- - ZrIII and SrIII, clearly demonstrated by XPS. For the effects seen in the mixed oxide ZrSiO4, the preferential depletion of SiO2 has been tentatively explained in terms of BIS phenomena, while the enhanced ZrIV reduction (with respect to pure ZrO2) is explained in terms of competing O–Zr and O–Si recombination reactions. © 1999 American Vacuum Society.