Annealing effects on silicon-rich oxide films deposited by plasma enhanced chemical vapor deposition are studied by spectroscopic ellipsometry. From the variations of the film thickness and optical functions caused by thermal treatments at two different temperatures the properties of the different films are derived. Hydrogen and OH release and structural reordering are recognized to be responsible both for thickness decrease and Si-Si bond formation. In samples far from stoichiometric SiO2?, the presence of Si-Si bonds after high temperature annealing accounts for both refractive index and extinction coefficient increase. Such effects are found to be directly linked to the silicon content in the films.