AbstractOxygen delignification can be considered to be the most important part of TCF and ECF bleaching sequences because it allows for cleaner production of pulp. During the process, oxygen gets one electron from lignin in the alkaline condition to form some active oxygen species (AOS), including a superoxide anion radical (O2-•), which is crucial for lignin degradation without damage of carbohydrates. The reaction of O2-• on cellulolytic enzymatic lignin (CEL) from Masson pine was studied. The change in active hydroxyl content after reaction with O2-• was investigated using 31P-NMR. After reaction, the aliphatic hydroxyl and uncondensed type phenol hydroxyl contents decreased, but the content of carboxylic group increased in Masson pine lignin. Through the analysis with HSQC-2D13C-H technology, β-O-4 linkages could be cleaved by O2-•, but β-β and β-5 linkages were observed to be more stable; benzaldehyde and cinnamic aldehyde structures could be oxidized to carboxylic acids by O2-•. Guaiacyl units in lignin were more easily degraded than p-hydroxybenzene units.