After a discussion of published evidence relating the rate of fading of a colorant to its physical state in the substrate and to the photochemical layer effect, the development of a mathematical model to relate fading rate of a colorant particle to its radius, when fading occurs by combined action of light and oxygen, is described. This model is in agreement with recently reported data on the fading of pigments, of measured particle size, in various paint binders. -- AATA