The correlation between natural weathering and different accelerated test methods has been discussed for many years; it is usually assumed that the rate of paint degradation is proportional to the intensity of the incident ultraviolet radiation. Results from studies of the photo-oxidation of isopropanol by titanium dioxide (TiO₂), a reaction known to correlate well with the weathering of paints, indicate that under the light intensities normally encountered, the oxidation rate is proportional to the square root of the light intensity. This conclusion is shown to be the end result of the processes occurring in the TiO₂ crystal when illuminated by UV light. The implications for the weathering testing of paints are discussed.