Post-deposition exposure to the ambient of SiO2 PECVD thin films is investigated by phase modulated ellipsometry. Real time and static measurements show evidence of changes in the structure of hygroscopic films due to water incorporation. These films show changes in the IR spectra associated with the development of the bands centered at 3450 cm(-1) and 950 cm(-1) assigned to OH and SiOH bonds. The evolution of the Delta = f(psi) curves exhibits two different kinetics and it appears that more than 50% of the incorporated water is present within a few seconds after the contact with air. If the exposure time does not exceed a few minutes, incorporated water molecules can be reversibly desorbed. Three models based on BEMA are proposed to account for the experimental data.