Photo-DSC was used, to study, in situ, the photo-ageing of polycyclo-octene which is a semi -crystalline elastomer. The 'crystallizability', which is the ability of the polymer to crystallize, was tightly dependent on the exposure time and was used to follow the photo-ageing. The irradiation system of photo-DSC was also compared with a usual accelerating device and no difference was detected in the chemical alteration of the polymeric matrix. Meanwhile, with photo-DSC, the photo-ageing was accelerated compared to a usual accelerating device. By using photo-DSC, atmosphere, exposure time, light intensity and ageing temperature were accurately controlled. (c) 2005 Elsevier Ltd. All rights reserved.