Interaction of sunlight and UV light with thermomechanical (TMP) pulps, milled wood lignin (MWL) and various lignin model compounds in solid state was studied. It was found that TMP fibers were sensitive to these lights. During exposure, TMP lost its brightness as a function of irradiation time and increased its yellowness. The α-carbonyl group functioned as a light absorption center to promote photocleavage of Cα—Cβand β–0–4 linkages. Electron spin resonance (ESR) studies demonstrated that free radicals were generated in the interim. Phenoxy radicals appeared to be important radical intermediates that ultimately transformed into o—quinonoid structures.