Photon-stimulated desorption (PSD) of positive ions from thick SiO2 films has been investigated by means of time-of-fight mass spectroscopy. From the untreated surface, H+, H-2(+), O+, OH+, H2O+, SiHx+ (x=0-3), and SiO+ were detected. Both soft x-ray irradiation and Ar+ ion bombardment reduced the H+ and O+ yields and increased the SiHx+ yield as a result of preferential removal of the oxygen-terminated sites at the outermost surface. The O+ and H+ PSD yields are correlated but the SiHx+ and H+ yields are not, indicating that most of the H+ ions originate from hydroxyl sites. Thermal treatment of the specimen at 800 degrees C, on the other hand, reduced the intensity of all the ion signals because of the structural change induced by condensation of H2O with the formation of surface siloxane bonds.