SiOx films with different oxygen contents were prepared by reactive magnetron sputtering and annealed at a temperature range of 200-1000 degrees C in ambient air atmosphere. The dependences of the film thickness, refractive index and optical gap, as well as the structural and compositional properties on the annealing temperature were studied. Three evolution stages with annealing temperature were observed. The first stage occurs below 400 degrees C, at which the film thickness expands, the refractive index reduces and the optical gap increases. The second stage sets in at around 400-500 degrees C and persists to a temperature of around 800 degrees C. At this stage, film thickness reduces, refractive index increases and the optical gap decreases. The third stage takes place at annealing temperatures above 800 degrees C, where the films become inhomogeneous from surface to interface due to oxidization. Stress relaxation, phase separation and oxidation mechanisms were correlated with the optical properties of the annealed films. (C) 2008 Elsevier B. V. All rights reserved.