The mechanism of photodegradation of cotton cellulose sensitised by sodium anthraquinone‐1‐ and ‐2‐su!phonates (A) has been examined. Measurements of diffuse reflectance indicate that the sensitisers are present in monodisperse form on the cotton cellulose and interact, on irradiation (λ, 365 nm), to give the corresponding anthrahydroquinones (AH2). A hydrogen‐abstraction reaction, involving the nπ*‐excited state (A*), leads to the formation of the semiquinone (AH·), which produces the anthrahydroquinone, probably by a further abstraction process:A* + Cell → AH· + Cell radicalThe accompanying decrease in degree of polymerisation of the cotton cellulose is equivalent to the production of AH2. The photoinduced changes in the sensitiser and substrate are first‐order in sensitiser concentration. Oxygen and water vapour are necessary for the cyclic regeneration of the sensitiser, which proceeds at the same rate for both 1‐ and 2‐sulphonates. However, sodium anthraquinone‐2‐sulphonate is 16 times more efficient as a sensitiser than the 1‐sulphonate, a result which is in agreement with the quantum yields for the production of AH2from