The contact angles of most surfaces modified by ion implantation change with the passage of time. The aim of this experiment is to better understand the mechanism leading to the aging of argon implanted silicon oxide surfaces. Coupons of silicon with its native oxide layer have been irradiated by means of 3 keV Ar ions with a fluence of 1.8×1016 Ar/cm2. Some of the implantations have been performed under oxygen partial pressure (∼5×10-5 Torr). The samples have been characterized at different periods of time after the implantation, by means of contact angle hysteresis measurements and angle resolved x-ray photoelectron spectroscopy (ARXPS). Irradiation with Ar ions has produced a more hydrophilic surface immediately after implantation. With the passage of time, the contact angles have increased both in ambient air and in argon atmosphere. Characterization by means of XPS has shown that there is with time a small increase of the O concentration as well as a small decrease of the quantity of Ar in the silicon oxide surfaces. The shape of the C 1s and Si 2p spectra and their details are also slightly modified with the passage of time. A model using the ARXPS measurements suggests that for the samples implanted without oxygen, the aging would be principally due to the spreading of a natural carbonaceous surface layer dispersed into islands after the Ar implantation. The layer being more hydrophobic than the silicon oxide, its spreading would increase the apparent contact angles. On the other hand, the increase of the carbon concentration in the carbonaceous layer would produce a more hydrophobic surface and would be responsible for the aging of the samples implanted in O2 partial pressure. Results obtained by means of a simple model and - the Cassie’s equation suggest that for the samples implanted without O2 gas, the contact angles of the substrate and the carbonaceous layer are 5° and 110°, respectively. The apparent contact angle obtained with the Cassie’s equation is in good agreement with the experimental measurements. © 2002 American Institute of Physics.