The effect of moisture upon free radical formation in celluloses irradiated with ultraviolet light has been studied by means of ESR spectroscopy at 77[degree]K. As studied from the variation of the line-shape and the relative signal intensity of the spectra observed, it is clear that moisture content in the sample greatly influences the formation of free radicals when irradiated with ultraviolet light. The moisture content in the range of 5-7% leads to a significant decrease of radical formation; and alternatively, when moisture content is lower or higher than this range, the formation of free radicals is increased.