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Mechanochemical reactions of ß-O-4 units of Hgnin were discussed on the basis of thc rcsults from veratryl glycerol-ß-guaiacyl (I) and vanillin (II) ethers and relatcd compounds, which wcrc trcatcd with ceramic ball mill for l hr in air. The homolytic cleavage of aryl cthcr bonds was obscrvcd only in thc compounds containing para -carbonyl group. In particular, the cleavage of Cß-O bond in compound (II) gave vanillin with 18.8% yield.This was followed by the homolytic cleavage of Ca-Cß to givc 3 and 4 and further by emission of formaldehyde to afford 7. However, condensation reaction among radicals also occurred to give 5 and o.These results suggested that mechanical pulp lignin containcd more amounts of para-carbonyl phenol groups compared to those in original wood lignin, and thcrcforc would givc significant effect on pulp bleaching with alkaline hydrogen peroxide.