A series of sweet gum kraft pulps with initial kappa numbers ranging from 29 to 13.6 was subjected to oxygen delignification employing 1.00 and 4.00% charges of caustic. The O-delignified pulps were characterized according to lignin content, pulp yield, and viscosity. In addition, residual lignin samples were isolated from the kraft brownstock and the post-oxygen-delignified kraft pulps, along with the corresponding O-effluents. The structural changes of these lignins during oxygen delignification were investigated using UV spectroscopy and advanced NMR techniques. Differential UV spectra indicated that lignins isolated from oxygen-delignified kraft pulp and its effluent were significantly reduced at lambda(max) 260, 280, and 370 nm in comparison to the brownstock lignins. These changes in absorbance are attributed to the degradation of phenolic and stilbene structures during oxygen delignification. P-31 NMR lignin data revealed that the primary sites of oxidation during an O-stage were syringyl and guaiacyl phenoxy groups. C-13 NMR analysis clearly indicated that the residual lignin from a low-kappa-number brownstock hardwood kraft pulp was enriched with guaiacyl units. These units are probably less reactive and accumulate with the extent of kraft cooking and oxygen delignification. The residual lignin from the post-oxygen-delignified pulps was also shown to contain enriched amounts of carboxylic acids and substituted aromatic carbons. The reduced oxygen bleachability of low-kappa-number hardwood kraft pulp is detrimentally influenced by the accumulation of inert guaiacyl units.