Ultraviolet radiation has been used to anneal out extrinsic defects in several types of deposited a-SiO2 films. The UV light was obtained from a new krypton UV-VIS-IR lamp with a spectral range of 170 nm <λ<3μ This “cold” annealing was performed on a-SiO2 films, with various thicknesses up to 400 nm. The films were deposited by various techniques, ultraviolet induced chemical vapour deposition (UVCVD), plasma enhanced vapour deposition (PECVD) and spin-on-glass (SOG). Fourier Transform Infra-Red spectroscopy (FTIR) and Electron Spin Resonance (ESR) were used to characterize the effect of the radiation. In the case of UVCVD and SOG a-SiO2 films, it is shown that the UV radiation removes the Si-H bonds and reduces significantly the amount of C-H and C-H3 groups. In both these films, an important reduction in the amount of adsorbed water and Si-OH groups is observed, together with an increase in the number of Si-O bonds. In PECVD films made with tetraethylorthosilicate (TEOS) vapour and O2 as precursor gases, we find evidence for an important reduction in the amount of C and the number of Cn Hy related defects. The UV treatment is effective even at temperatures as low as 100 °C, which suggests that it could constitute a much needed low temperature annealing step.