This paper focuses on the description and evaluation of aging mechanisms in multi-layer films that are used for encapsulation of photovoltaic (PV) cells. The materials investigated include polyethylene terephthalate (PET), polyvinyl fluoride (PVF) and polyvinylidene fluoride (PVDF). The materials were exposed to a relative humidity of 85% at 85 degrees C ("damp heat test"). To describe the aging mechanisms, materials were characterized by infrared spectroscopy in attenuated total reflection mode (ATR), by differential scanning calorimetry (DSC) and by mechanical tensile tests. The damp heat test resulted in significant hydrolysis of PET and post-crystallization of materials. Tensile tests showed a significant embrittlement and thus chemical degradation of PVDF and PET caused by the damp heat test. The strain at break value proved to be a very sensitive indicator of global aging. (c) 2005 Elsevier Ltd. All rights reserved.