A reaction mechanism for the photodegradation of cellulose was proposed on the basis of the results obtained in an investigation of the photodegradation of cellulose in cadoxen. The influences of the individual components of the complex solvent were studied by observations of the photodegradation of cellulose derivatives in the individual components of the solvent. The primary photolytic scissions were considered to be distributed between the glucosidic and C₁-C₂ bonds of the polymer. In the absence of oxygen or the presence of a suitable complexing agent the quantum yield is thought to be principally determined by the number of glucosidic bond scissions. In the presence of oxygen and the absence of such an agent both types of initial bond scission contribute to chain scission and the quantum yield is thought to be principally determined by the number of C₁-C₂ scissions. The quantum yield for the photodegradation of cellulose in cadoxen was found to be 1.70 x 10* h-3*s. This figure is in reasonable agreement with the values obtained by other workers for the solid state degradation. The greatly increased quantum yield values observed for the photodegradation of cellulose derivatives in the presence of oxygen in non-complexing solvents is thought to indicate that the number of C₁-C₂ bond scissions effective in reducing the degree of polymerisation of the polymer is strongly dependent on the surrounding medium.