The delignification kinetics and delignification/carbohydrates degradation selectivity of oxygen delignification (O-delignification) of a high kappa number (HKn) pine kraft pulp (kappa 65) were studied at different alkali concentrations (1-4 g NaOH/L), temperatures (95-105 degrees C), oxygen pressures (5-10 bar), and reaction times (5-90 min) in a flow-through (FT) reactor. This continuous muted batch reactor operating at constant and relatively low alkali concentration allows degree of lignin removal greater than 70% at higher yield (about 2% on wood) where the final kappa number is similar to 15 equal to that of oxygen-delignified conventional kraft pulp. In addition, the effect of operational conditions on delignification, cellulose degradation, and pulp yield are compared to that of oxygen-delignified conventional kraft pulp.