The peel test in the photovoltaic (PV) industry is a widely used method to characterize adhesive properties of materials used as encapsulants for PV modules. This thesis deals with the various factors that influence the peel strength determined from peel tests. 180° peel tests and T peel tests were carried out on specimens laminated from glass, 5 different encapsulants and 3 different backsheets. Peel tests were conducted at different temperatures (23 °C, 45 °C, 70 °C) and test rates (5 mm/min, 10 mm/min, 50 mm/min, 100 mm/min) and different peel arms were used in 180° peel tests. Half of the specimens were aged for 1000 h in damp heat conditions. While evaluating the peel tests, two major problems arose. In many cases the crack did not propagate at the desired interface and specimens did not even break in the weakest layer. This emphasizes the necessity of documentation whether the rupture was cohesive or adhesive and at which interface. The second problem was the wide scattering of peel strength, stressing the importance of equal conditions during fabrication, storage and testing of specimens. For the wide scattering, no significant influence of the peel arm’s stiffness on the peel strength was identified. The stiffness of the encapsulant, however, turned out to be crucial for the kind of fracture. The higher the encapsulant’s stiffness, the more prevalent was delamination within the backsheet instead of an adhesive fracture between the encapsulant and glass or backsheet because high stiffness of the encapsulant leads to a smaller peel radius and therefore higher local stresses in the backsheet. In general, peel strength for adhesive and cohesive fracture decreased with rising test temperature. Some materials showed stronger temperature dependence than others. Contradictory to time-temperature equivalence, the test rate hardly showed any effect on the peel strength. Only for cohesive fracture was a slight increase of peel strength with rising test rate observed. For all encapsulants the peel strength decreased after damp heat aging. Again, as for temperature dependence, the effect was individual for each material. For one of the encapsulants the prevalent fracture mechanism changed after aging: aged specimens broke via cohesive fracture within the encapsulant close to the interface to glass, exhibiting much lower peel strength than for a cohesive fracture deeper within the material, suggesting the creation of a weak boundary layer.