The effect of dye and dye concentration upon the photodegradation of a cotton fabric was determined for five trichloropyrimidine reactive dyes (Drimarene X, Sandoz). After irradiation in a xenon arc Fade-O-Meter, the dyed cotton substrates were found to be less degraded than the undyed sample. This protection increased with an increase in dye concentration. Similarly, this protection was better when the dyes were lightfast, except for C. I. Reactive Green 15 (good protection, poor lightfastness). However, C.I. Reactive Blue 52 was found to accelerate degradation to the cotton substrate. Its damaging effect increased with dye concentration. -- AATA