Formation of free radicals in photoirradiated cellulose has been studied by means of ESR spectroscopy at 77°K. Three kinds of light sources with different wavelengths between 2500 and 4000 Å were employed. No radicals detectable by ESR were recorded when cellulose was irradiated with light of wavelength longer than 3300-3400 Å. Hydrogen atoms that generated a doublet spectrum (DELTAH = 508 G) were observed when cellulose was irradiated with light longer than 2800 Å. Hydrogen atoms and formyl radicals that generated doublet spectra with splitting constants of 508 and 129 G, repsectively, were observed when irradiated with light shorter than 2800 Å. The scission of the polymer chain in cellulose is evident from decrease of the degree of polymerization, and the results of mass spectometric analysis indicated H2, CO, CO2, and H2O to be the main volatile products of cellulose upon photoirradiation.