AbstractEnhancement of oxygen delignification is critical to improve subsequent bleaching efficiency while being environmentally compatible. In the present study, xylanase was used to improve the delignification process of poplar kraft pulp. Results showed that the kappa number reduction ratio (KRR) of 14.5% was achieved for the pulp under xylanase-assisted oxygen delignification processes when compared to the control without xylanase treatment. Other pulp properties, such as intrinsic viscosity and brightness, also improved somewhat; i.e., viscosity increased by 28 mL/g units and ISO brightness increased 1.4% points. Furthermore, 31P-NMR was employed to characterize the chemical structure of the residual lignin of the pulps before and after oxygen delignification. It showed that the condensed phenolic and syringyl hydroxyl groups decreased significantly for the xylanase-assisted oxygen-delignified pulps.