The effects of humidity and oxygen on the photofading of Crystal Violet(CV) in nylon 6 film were studied in detail. Both moisture and oxygen were necessasy for the formation of Michler's Ketone. The effects of temperature and humidity on the photofading of CV in nylon 6 and polyester films were discussed in terms of kinetics. The fading rate was accelerated with rises in temperature and humidity for both films, and the rate was lower in polyester film than in nylon. The quantum yield in film decreased with the progress of decay, whereas it was almost unchanged in solution. The relationship between relative quantum yield( ø ) in film and absorbance(A) at λmax was expressed by the equation, ø = β (A — a0)m. The parameter m indicates the ease of decrease of the rate with the progress of decay. The influences of temperature and humidity on the value of m were compared. The value of m depends on the initial absorb-ance in some cases. At elevated temperatures m has smaller value in almost all the cases. © 1990, The Society of Fiber Science and Technology, Japan. All rights reserved.