Decomposition of the Si-O-Si bulk network and the composition change in vacuum ultraviolet (hv > 100 eV) irradiated SiO2 films have been investigated by spectroscopic ellipsometry and transmission electron microscopy. Real-time monitoring the dielectric function revealed that the irradiated SiO2 film initially evaporates while accumulating Si-Si bonds and that when the evaporation stops, further volume loss is primarily due to the formation of oxygen vacancies. Finally a steady-state composition is reached because the increased valence electrons made available as a result of the formation of Si-Si bonds efficiently quench the electronically excited state. At irradiation temperatures lower than 300 degreesC, the Si-Si bond units are distributed randomly within the SiO2 matrix. Between 470 degreesC and 690 degreesC phase separation of SiO2 into Si and SiO2 domains yields Si nanocrystals embedded in the SiO2 matrix. This results in the volume fraction of SiO2 being between 80% and 0%. Above 700 degreesC, the SiO2 domains disappear, leaving Si crystalline islands at the surface.