The yellow colouration that occurs when a cellulose material is exposed to heat and light is considered to be an indication of its degradation. By using diffuse reflection spectra techniques, we have been able to determine the absorption bands that appear when cellulose is heated in darkness. We have also been able to demonstrate the reversibility of the colouration effect, as this disappears if the cellulose material is exposed to daylight at ambient or low temperature. Photorestoration can be used for obtaining a higher degree of whiteness than in the initial state; in addition, the colouration effect returns if the unprocessed cellulose initially subjected to light action is placed away from light (post-irradiation effect) and it is more intense if the temperature is increased. The bleaching of cellulose materials by light has been remarked by several research workers, but, to our knowledge, the photochromic nature of these colour reactions was not demonstrated following these observations. During the course of our investigations, we have noticed that this phenomenon occurs for various cellulose materials whether it be lignified cellulose such as newsprint or cellulose in the most pure form we have been able to produce.