In certain circumstances, the light fastness of textiles may be a factor of considerable importance. However, general statements as to their mechanical behaviour under exposure to light are not possible, since too many influencing factors are at work simultaneously (sunlight, atmospheric pollution, temperature, rain, etc.). Using a mathematical model (Weibull function), attempts were made to obtain from laboratory tests certain indications as to the behaviour of fibres under actual conditions, making a number of assumptions for the sake of clarification. Using eight textile fibre yarns, eight different exposure methods were then experimentally compared with open-air weathering. It was found that the laboratory methods, when applied during a set period of time, correspond to 1 to 100 days of open-air weathering, depending on the type of fibre. None of the methods tested showed any particular advantages.