The stability of four types of colour transparencies is discussed, for storage at room temperature, storage at elevated temperature and relative humidity (RH), and for accelerated light ageing. When complicating factors such as heat and RH sensitivity and colour recovery in the dark are taken into account, the films can be ranked: Kodachrome, then Fujichrome, have the best dark stability, while Fujichrome and Ektachrome have the best stability to light. Agfachrome showed loss of colour after eight year is the dark. Underexposed and slow films last longer, and RH affects storage stability more than temperature. All the films have a lightfastness worse than ISO Blue Wool Standard 4, unless protected by an ultraviolet filter. -- CAL