Cellulose and its derivatives -- cellulose acetate -- are renewed polymers, that, together with the whole complex of valuable and indispensable properties, defines continuous growth of their production. Acetate fibres differ from cellulose fibres in light and thermooxidative stability, as the presence of ester groups decreases stability of molecular structure, owing to which destructive processes begin at much lower temperatures and weak energy effects. Since macromolecules of cellulose acetate are constructed on the basis of cellulose then mechanism of photodestruction of these polymers may be considered as general. The most important energetic factor, which photodestruction of cellulose and its derivatives depend on, is intensity of irradiation and wave length. Destruction of cellulose and its derivatives under atmospheric conditions, proceeding as a result of photochemical reaction, on the whole takes place under the action of ultraviolet rays with ?=200-360 nm. Since cellulose contains three types of chromophore groups -- hydroxyl, acetate and semiacetate and also aldehyde -- then it is considered that light absorption in the region of 250-300 nm is caused just by them. At the same time some authors, bringing the possibility of light absorption by acetal chromophore in question, have put forward the supposition that photochemically active centres in cellulose materials, containing carboxyl and hydroxyl groups, may be molecular complexes between these groups, connected by the system of hydrogen bounds with definite energy of interaction. Disproportion of intermolecular bonds, providing fixation of excited state in cellulose matrix takes place in such complexes at their excitation. Thus, there are many different hypotheses, often contradicting each other, about the effect of chromophore groups on light absorption by cellulose. There are many data about the nature of free-radical particles, being formed at irradiation of cellulose by ultraviolet light. Since, being formed products of phototransformation are highly mobile and easily undergo further transformations, method of electron-paramagnetic resonance (ESP) is one of the most effective for these particles identification. Critical analysis of a great number of works on EPR spectra interpretation is quite fully given in surveys. More than 20 different radicals are being formed at ultraviolet irradiation as a result of break of practically all bonds C-C; C-H; C-O. Main types of macroradicals, with indication of atom and groups of atoms after removal of which these macroradicals are formed, are presented in scheme 1. of this book.