The effect of sensitising dyes on the photochemical degradation of textile fibres is discussed. The degradation is dependent upon the presence of oxygen, and is accelerated by water vapour and rise in temperature. The study of simple model photochemical systems intended to simulate the dyed fibre system indicates that there are significant differences between the two systems. The sensitising effect of dyes in the textile systems seems to be due to the formation of excited singlet oxygen and hydrogen peroxide. The latter is not formed in the absence of water vapour.