Infrared and Raman spectroscopy, thermal analysis, electron scanning microscopy and mechanical tests have been performed to study the outdoor ageing of Carilon (a terpolymer which can be described as an ethylene-carbon monoxide alternate copolymer containing a small amount of propylene units). The results of these analysis are discussed and compared to obtain a description of the structural modifications induced by photodegradation. All the experiments performed show evidence of relevant structural changes in the material localised on the surface of the samples: chemical modification of the polymers, breaking of polymer chains and rearrangement of the crystalline phase can be clearly identified. Moreover, mechanical tests reveal that outdoor ageing involves to some extent the bulk of the material, with a consequent loss of mechanical properties. (C) 2001 Elsevier Science Ltd. All rights reserved.