The lightfastness of some azo dyestuffs was evaluated using kinetic parameters. The rate of photodegradation of the dyestuffs was determined in the presence of radicals generated from isopropanol in the liquid phase. Also, radical processes were produced with dyestuffs on cotton. The lightfastness of the dyestuff was characterized by using kinetic parameters that determine the radical-forming ability of the dyestuff and its reactivity toward radicals.