The present study explores the efficiency of delignification achieved during standard oxygen, double-oxygen, and mini-oxygen [(E + O)D-kf=0.05(E + O)] delignification of high- (kappa = 56.2) and low- (kappa = 26.6) lignin-content softwood (SW) kraft pulps in the context of the structural changes occurring in the lignin as measured by nuclear magnetic resonance (NMR) spectroscopy. The relative bleachability of the high-kappa pulps was determined to be superior to that of the lower-kappa pulps during the oxygen bleaching experiments. The general trend of increasing ease of bleachability was double oxygen > oxygen > mini-oxygen for both the low- and high-lignin-content pulps. NMR spectroscopy demonstrated that part of the rationale for the higher levels of delignification in the high-kappa pulps was due to higher contents of beta-O-4 and methoxy lignin functional groups. In addition, the high-kappa pulp contained a lower number of resistant 5,5'-condensed lignin units and diphenylmethane structures. The NMR data also provided strong evidence for the presence of p-hydroxyphenyl units, a relatively unique resistant structure whose elimination was approximately the same for both series of pulps. These new structures might potentially function as end-capping termini that hinder access to bulk lignin clusters, thus limiting the overall efficiency of oxygen delignification.