The degradation study of photovoltaic modules is a subject of great interest for the industry sector. Although the degradation of PV cells has been previously studied [1, 2,3,4], these estudies has been focused on the explanation of semiconductor material degradation. There are other causes such as glass breakage (front and back of the module), oxidation, junction box connections corrosion, delamination and bubbles formation in the encapsulant, back sheet polymer cracks, etc. The manufacturing process of photovoltaic modules is very standardized. Most are formed by a sandwich clear glass, low iron content, ethylene vinyl-polymer acetate (EVA), cells, EVA and backsheet (usually Tedlar-Polyester-Tedlar or only one film of polyester). The use of EVA as encapsulant polymer allows maintaining a good optical transmission in a prescribed spectral region providing high visible transparency, excellent mechanical properties, good electrical isolation, etc. However, EVA is one of the polymer materials developed for the encapsulation of crystalline Si-based PV modules.EVA film presents different degradations along the time: it can develop a slow degradation from light yellow to dark brown colour. However delamination and bubbles formation are the most concerning problems related to EVA. They are related not only to the bad quality of EVA but also to a bad process of lamination, where the cross-linking process has not been well performed. Due to ageing and temperature changes the EVA degradation can lead to a high power loss of the module or even its rejection. The tests for this work were performed according to the recommendations of the UNE-EN 61646:2008 «thin-film photovoltaic modules for terrestrial use», with the necessary adjustments in order to apply to a single cell. The set of glass films and EVA were soaked to ultraviolet (UV) radiation at different exposure times. By optical analysis a correlation between degradation, time and EVA thickness has been observed. Defects like EVA delamination due to a lack of adherence and colour change were detected by visual inspection and they were found to be related to exposure time.