Lightfastness of simple monoazo dyes on nylon 6 fabric has been studied. Examination of the fastness-grade curves provides information about the relation between dye-particle size distribution on the fiber and dye concentration. The fading rate of the dyes as a function of irradiation time (daylight and carbon-arc Fade-Ometer tests) leads to evaluation of the fadingreaction order. The shapes of the fading-rate curves and the characteristic fading curves are used to diagnose the physical state of the dyes in the fiber. -- AATA