AbstractIn an attempt to explain variations in delignification behaviors among different hardwood species, the kraft pulping delignification rates of Eucalyptus urograndis, E. nitens, E. globulus, sweet gum, maple, red oak, birch, red alder, cottonwood, and acacia were obtained and correlated with their respective lignin chemical structures. Since H-factor for hardwood is calculated based on the softwood activation energy (Ea) value, a comparison between softwood vs. hardwood activation energy was also performed. Lignin was isolated by a modified isolation protocol, using alkaline pretreatment of the wood prior to isolation. The lignin preparations were analyzed via quantitative 13C NMR spectroscopy. Substantial variations were found among the hardwood species studied. A linear correlation between the kraft delignification rate and the amount of syringyl units was found. Activation energy values obtained for kraft pulping of hardwoods were very similar and almost identical to the value obtained for softwood. Birch was the only species with outlier behavior.