Abstract In this paper qualification tests of various plastic lenses for operation in ultra-high vacuum (UHV) are reported for the first time. The results of a systematic investigation, covering the temperature range 20–200 °C and aimed at qualifying the resin ADC200, an allyl diglycol carbonate, as a window for UHV vacuum vessels, are described. Tightness and permeation tests were performed and optical transmission monitored in the interval 200–850 nm, to assess the effects of both the UHV environment and thermal excursion. Compatibility with UHV operation was checked independently over the temperature range 40–350 °C using mass spectrometry. The results show that the polymer does not emit any contaminating components incompatible with UHV systems. Mechanical integrity was shown to be guaranteed up to a temperature of 150 °C and no sign of degradation in the optical properties measured was ever detected. The major drawback of this technology for UHV would appear to be the high rate of He permeation through the plastic material. It is possible that this could be remedied by the application of suitable coatings.