This paper reports the application of a previously described i.r. spectrometric method, which measures the evolution of carbon dioxide, and hence the rate of photo-oxidation, caused by UV irradiation of paint films, to a study of the dependence of paint degradation on UV wavelength. Measurements have been made on both unpigmented and TiO2-pigmented acrylic films. Preliminary measurements demonstrate that, for both pigmented and unpigmented acrylic films, the rate of carbon dioxide generation is proportional to I-0.5, the square root of the UV intensity. As the rate of carbon dioxide generation is a measure of paint film degradation, this implies that the rate of film degradation is proportional to the square root of UV intensity. This is the first time that this square root dependence, previously predicted from photo-oxidation studies of model compounds, has been directly measured on a practical paint film. Photo-oxidation of unpigmented, anatase-pigmented and rutile-pigmented acrylic films has been measured by using optical filters to isolated selected portions of the output of a Xenon lamp. Results have been analyzed semi-quantitatively taking into account the measured transmission characteristics of the filters, the lamp output and the I-0.5 relationship. The disproportionate deleterious effects of short wavelength, < 300 nm, radiation on unpigmented films have been confirmed and the stability of anatase pigmented films to 405 nm radiation has been demonstrated directly. The FTIR assay of carbon dioxide produced by paint film degradation could be applied to other polymer films and combined with measurements of carbonyl band development.