AbstractUnstabilised PVC has been photo‐oxidised by exposure to xenon‐arc radiation from which shorter wavelengths had been removed by a series of filters. The wavelength sensitivity was found to correspond approximately to the complete ultraviolet region of the solar spectrum, maximum photo‐oxidation rates occurring at about 370 nm. Solar radiation data for Melbourne have been presented to show that variation in the ultra‐violet component with time of day and season is unimportant, to the extent that global radiation data may be utilised to characterise an exposure site with respect to the rate of photooxidation of PVC at th