FTIR assay of UV generated gaseous CO2 has been used to monitor the photo-degradation of alkyd paint films in oxygen of controlled humidity. The three contributions to CO2 evolution from such paint films are (a) a dark reaction associated with the drying process, (b) direct photochemical reaction, and (c) photocatalysis by the TiO2 opacifier. As this paper is focused on the photocatalysis by TiO2, the dark reaction has been minimized by accelerated drying of the paint films and the photocatalytic component has been emphasised by studying films pigmented with TiO2 that has not been surface treated. This methodology allows the UV stability of alkyd paints to be monitored rapidly by an FTIR measurement of the UV-generated carbon dioxide. CO2 evolution was greater for films made with high (35%) than for low (15%) TiO2 pigment volume concentration (p. v. c.) paints. This behaviour, which parallels weight losses from similar films exposed in conventional accelerated weathering equipment, shows that photocatalysis by the TiO2 is dominant. The CO2 evolution rates from films opacified with different grades of rutile pigment correlate with weight loss measurements made from identical paints exposed to glass-filtered carbon arc irradiation in conventional accelerated weathering equipment. These two comparisons confirm the relevance, to conventional accelerated testing, of paint durability of the FTIR assay of carbon dioxide evolution from alkyd paints. Because the experimental procedure allows the ready interposition of suitable optical filters, the method is well suited to studies of e. g., incident wavelength & intensity. In addition, deliberate variations in the humidity of the atmosphere in the FTIR cell were monitored by using the i.r. absorption of water vapour. Not only did the observed rates of carbon dioxide evolution decrease with decreasing humidity, they extrapolated to zero at zero humidity. This is consistent with mechanisms, derived from studies of model systems, that identify hydroxyl radicals as the key catalytic intermediate in photocatalytic oxidation of TiO2 pigmented paint.