This study presents findings on the effect of domestic laundry detergents on the fastness to light of selected fibre reactive dyes applied to cotton. The study was carried out to elucidate the reasons for the accelerated colour loss of cotton garments washed under Australian domestic laundering conditions. Cotton fabric dyed with commonly used reactive dyes were laundered with water only, several domestic detergents and a laboratory formulated neutral detergent, and then exposed to light for two hours in the wet state. Quantities of detergent used were in accordance with the manufacturers recommendations. Exposures were repeated fifteen times equivalent to 30 hours exposure. Colour loss and colour difference were measured after 5, 10 and 15 wash cycles and 10, 20 and 30 hours exposure. When the fabric was exposed wet the colour faded more rapidly than when exposed dry to light. Detergents increased the colour loss even when the fabric was not exposed to light. The presence of an oxidizing bleach (sodium perborate) in the detergent increased colour loss during washing and wet exposure to light. Ultraviolet radiation from the light source, heat, moisture, alkali and oxidising bleach during exposure resulted in hydrolysis of the dye-fibre bond causing dye desorption during washing and rinsing. Water alone increased the fading of the dyes most likely due to presence of dissolved oxygen held within the fibre. The combination of ultraviolet radiation and oxidizing bleaches altered the chemistry of the dye and hence its shade. This was particularly evident on the black dye and one of the navy blue dyes.