Room-temperature photostructural changes induced in high-carbon a-Si:C:H films prepared from silane and propane were monitored by examination of the behaviour of the Si 2p, C 1s and O 1s XPS. Silicon oxide is produced by UV illumination in oxygen, although interstitial oxygen does not support the process, and C-Si, C-C and C-H bonds are unaffected. The growth rate is a log function of the illumination period, consistent with the high microvoid content of the most easily oxidized material. Oxidant can move through the void structure to the site of oxidation, instead of the more usual diffusion-limited transport which would give a square root dependence on illumination period. © 1995 Elsevier Science B.V. All rights reserved.