AbstractAn outline of the causes and nature of fading of organic colouring matters is given. Atmospheric fading, in absence of light, is produced by nitrogen oxides in urban air, especially with blue and violet disperse dyes on some hydrophobic fibres, but sulphur dioxide and ozone can also cause fading. Light fading occurs with all dyes; some of the factors which affect it are discussed, including the physical state of the dye, the chemical and the physical nature of the fibre, the temperature and humidity of the atmosphere, and the quality and intensity of the illumination. The hue of many soluble dyes becomes redder on fading. The types of fading rate are described; the fading of many dyes is rapid at first, but later it slows to a constant rate, so that a dye, which by normal testing methods has low fastness, may yet withstand prolonged exposure.