The formation of porous silicon dioxide films on Si (100) and quartz substrates at low temperatures (25-300 o C) by photo-induced sol-gel processing using 172nm radiation from an excimer lamp has been investigated. The effects of substrate temperature, total gas pressure and lamp exposure time on the properties of the film formed have been studied using ellipsometry, UV spectrophotometry and Fourier transform infrared spectroscopy (FTIR). The FTIR spectra revealed a Si-O-Si stretching vibration peak at 1070cm - 1 after UV irradiation at 200 o C which is similar to that recorded for oxides grown thermally at temperatures between 600 and 1000 o C. The films exhibited optical transmittance of around 90% in the visible and UV region of the spectrum. Capacitance measurements indicated that the dielectric constant of the films strongly depended on the substrate temperature during irradiation. Dielectric constant values as low as 1.7 are readily achievable at room temperature.