Abstract: Unbleached softwood kraft pulp was subjected to low consistency oxygen bleaching. Total KMnO₄ consumptions by lignin in both pulp and effluent were measured and the difference of this value from the KMnO₄ consumption by lignin in original unbleached kraft pulp was used as the indication of the extent of oxidation that lignin in the pulp experienced during oxygen bleaching. Extent of lignin oxidation thus estimated well correlated to the delignification. One lignin unit was found to be oxidized by about 3 electrons as an average when Kappa number of the pulp became about half （from 25.4 to 13.4）. Methoxyl determination of the bleached pulp suggested that the residual lignin still remaining in the oxygen-bleached pulp was not extensively oxidized. These results confirmed quantitatively that lignin oxidation is essential for the delignification during oxygen bleaching.

Key words: lignin oxidation, delignification, oxygen bleaching