Post-irradiation darkening at room temperature of water-leaf papers made from various pulps is shown (through a series of experiments using three different sources of visible and ultraviolet radiation) to be chiefly influenced by the content of lignin and by the PH of the test sheets. The effects of different exposure dosages, initial PH values in the sheet, and levels of hot-alkali-soluble matter, gamma cellulose and lignin are presented. 15 references.