A study has been made of photochemical degradation of cotton, mercerized cottons, partially acetylated cotton, and partially cyanoethylated cotton, during exposure to carbon-arc radiations under different conditions of atmosphere in the system and during exposure to selected wavelength bands in the visible and uv of sunlight. Exposed fiber substances have been examined for both mechanical and chemical damage suffered, and explanations have been put forth for the behavior of modified cottons on the basis of the nature of the modification effected through different treatments.