Yellow‐browning of the ethylene‐vinyl acetate (EVA) copolymer encapsulant used in PV modules has resulted in significant power losses of over 50% of the initial power output. The weathering‐degraded yellow‐brown EVA films have lost the ultraviolet (UV) absorber, Cyasorb UV 531(R), and the degree of cross‐linking (gel content) has increased. EVA degradation mechanisms identified thus far are discussed in this work. Upon exposure to UV light at 45°–85 °C, virgin EVA films that are stabilized with Cyasorb UV 531(R)and two antioxidants show an increase in the gel content, a gradual loss of Cyasorb by photooxidation, and the generation of acetic acid. The deacetylation reaction, which leads to the formation of polyenes, also occurs significantly in the films heated in the dark at 130 °C for five days. Acetic acid thermally catalyzes the EVA film discoloration at 85°–130 °C, which increases from a light yellow to a yellow‐brown color as the heating temperature increases. The factors can account for the yellow‐browning of the EVA in the accelerated testing of mini‐modules, and the discoloration is more profound when exposed to UV light at 85 °C than when heated in the dark at the same temperature. In the presence of the EVA‐produced acetic acid, oxygen, and sunlight exposure, the Cu buslines that were coated with a thin layer of Pb‐Sn alloy showed significant oxidation and metal interdiffusion, which in turn may contribute to the resistance increase and hence the current loss reported for weathered PV modules.