After five or more years of weathering, the degradation of ethylene-vinyl acetate (EVA) encapsulant in photovoltaic (PV) modules resulted in a yellow to dark brown color. Degraded EVA shows a substantial increase in the gel content and a large to complete loss of the ultraviolet (UV) absorber, Cyasorb UV 531. The EVA discoloration is caused by the formation of polyconjugated (C=C)/sub n/ double bonds of various lengths. Acetic acid and other volatile organic components are also produced from the photothermal decomposition of the EVA. The solar cell efficiency was reduced by approximately 9% by a light-yellow-brown EVA and approximately 50% by a dark-brown EVA. Weathered PV modules with dark-brown EVA also show a approximately 50% decrease in efficiency.<<ETX>>