ESR spectra of purified and ferric ion-sensitized cellulose irradiated with ultraviolet light in vacuo at 45, 20, -80 and -196C were recorded and compared. Generally, several kinds of spectra, viz., singlet, three-line, five-line, and seven-line spectra, were observed. At higher temperatures, only singlet and three-line spectra of stable free-radical species were detected, whereas at lower temperatures such as at -196°C, two doublet spectra of formyl radicals and hydrogen atoms were also detected in addition to cellulose radicals. It is believed that the intricate spectra observed at low temperatures are superimposed upon spectra generated by free radicals which may or may not be stable at high temperatures. During reiiradiation at -196°C with an alternative light sources, i.e., lambda Ü0 2537 Å and lambda Ü0 3400 Å, of samples which were irradiated at 20°C or at -196°C, phenomena indicative of radical transformation and formation of new radicals or of decay of radicals in terms of ultraviolet bleaching were observed on studying the changes of line-shapes and relative signal intensitites of the spectra.