The degradation of amorphous poly(ethylene terephthalate) sheet material is investigated under various environmental conditions [dry, wet soil, 100% and 45% relative humidity (RH) and u.v. irradiation] by measuring the rate of chain scission using viscometric analysis at several temperatures. Using the Arrhenius expression, the lifetime of the polyester is found to be very dependent upon the environmental conditions, with hydrolysis being a dominant process at lower temperatures. Negligible degradation is observed at temperatures below the glass transition (~ 80 Β°) in dry conditions. From density measurements at 45 and 100% RH, the crystallinity exhibits an initial facile increase due to plasticization by the moisture and annealing followed by an inflection which increases from 26 to 40% with the severity of the degradation conditions. This inflection is consistent under all degradation conditions at 0.5 of a chain scission and is then followed by a much lower rate due to combined hydrolytic degradation/ oxidation of the polymer chains. The initial rapid increase is found to be faster at lower rates of chain scission because of an annealing/plasticization effect by the moisture. This view is confirmed by the fact that under both dry conditions and u.v. irradiation, where significant rates of chain scission are observed especially at high temperatures and prolonged times (> 500 hr), the crystallinity only increases to about 5%. The implications of these results are discussed in terms of assessing the degradation and hydrolysis of the polymer through physical and mechanical analysis in relation to actual polymer breakdown.