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KeywordsLignin Photodegradation β-Ο-4 Lignin modcl compound Phenolic dimer Photochemical bond cleavage 5,5' -O-4Tetramer Frec-radicals α-Carbonyl-phenyl derivatives synthesis Bisulphite pulp (maritime pine) SummaryThe photoyellowing of bleached high yield wood pulp induced by lignin was studied using the title phenolic a-carbonyl -O-4 lignin model L Its photochemical reactivity was examined in both liquid (benzene and buffered aqueous ethanol) and solid (adsorbed on bleached chemical pulp) media and compared to the 4-O-methylated dimer 2. The influence of oxygen during the Irradiation was considered. Efficient photolysis of l (0 r ~ 0.25 in solution) yields monomers, dimers, a tetramer and a complex mixture of oligomers which include coloured substances.The structure and the titration of the photoproducts indicate that an intramolecular β C-O bond cleavage mechanism leading to phenacyl and guaiacoxy radicals is the major pathway of the photodegradative reaction. The phenolic hydrogen abstraction mechanism, evaluated by the formation of the 5-5' tetramer 14 contributes only for 10%. These results support the previous hypothesis ) concerning the -cleavage of α-carbonyl β-Ο-4 dimer units s the primary step in lignin photodegradation.