The degradation of poly(ethylene terephthalate) (PET) (as substrate for audio-visual materials, amorphous sheet and bottles) has been studied by both accelerated thermal and photo-ageing methods. The degradation has been considered from the opposing viewpoints of environmental acceleration of degradation and prolongment of archival lifetime. In the former case, samples of both polyester sheet and bottles have been aged in contact with dry and wet soils in dark and light conditions at different temperatures to emulate environmental breakdown. In the latter case, non-processed 35 mm cinematographic film has been aged at various relative humidities and temperatures in contact with film containers (metal can) to emulate archival storage conditions. Results of both accelerated ageing studies indicate that breakdown of PET motion picture film is negligible at 60°C and relatively unaffected by variations in humidity of the surrounding environment, over the time period studied (300 days), due to its high crystallinity (55%). At 70 and 80°C the motion picture film exhibits signs of crosslinking rather than degradation due to the high crystallinity and emulsion inhibiting the diffusion of oxygen into the polymer. The presence of iron from the container has an accelerating effect on the degradation rate of motion picture film material but only at temperatures above 90°C. In contrast, normal amorphous polyester sheet and orientated bottles degrade due to their much lower crystallinity (1 and 30%, respectively) and at higher temperatures (70–90°C) breakdown, as characterized by viscosity and chain scission measurements, is indicative of significant polymer deterioration. Breakdown is enhanced by increasing temperature, increasing relative humidity and UV irradiation. The polyester bottles are more stable than sheet due to a greater degree of orientation in the former case and hence higher degree of crystallinity. Both soil (in the case of amorphous PET sheet and bottles) and metal storage can (in the case of cinematograph film) have a significant effect on stability. At temperatures above the glass transition, i.e. 80°C, differences in rates of degradation up to 45% relative humidity are not significant. Videotapes of various archival histories have also been studied using high resolution light microscopy. White crystalline deposits and surface conglomerates are observed with increasing age and these appear to be consistent with artificial ageing experiments.