The hydroprocessing reactions of kraft lignin and four of its model compounds, o-hydroxydiphenylmethane (OHD), diphenylmethane (DPM), 4-phenoxyphenol (PP), and 4-methylguaiacol (MG) were studied over a sulfided CoMo/γ-Al2O3catalyst. The reactions of OHD were also studied over sulfided Ni-Mo/γ-Al2O3, Ni-W/γ-Al2O3, and Ni-Mo/zeolite catalysts. Reaction pathways, kinetics, and catalyst deactivation were resolved. Experiments with the actual kraft lignin substrate showed that catalytic hydroprocessing led to higher yields of single-ring products and lower yields of light gas compared to hydropyrolysis alone. The CoMo/γ-Al2O3catalyst increased the selectivity to non-coking monophenolics and hydrocarbon products. The model compound results permitted interpretation. The hydroxyl substituents on OHD and PP directed bond cleavage strongly. The atom linking the two aromatic rings remained almost exclusively with the unsubstituted ring after cleavage. The substituent increased reactivity by at least an order of magnitude. The catalytic reactions of OHD, DPM, and PP were 103-104times faster than their thermal reactions. Deoxygenation was appreciable in these reactions and in the catalytic hydroprocessing of MG. Collectively, these results suggest that the improved activity and selectivity in catalytic lignin liquefaction is due to enhancement of link cleavage and the transformation of coking dioxygen-substituted phenolic coke pecursors (e.g., catechols and guaiacols) to non-coking phenols. Hydrotreating catalysts with alumina supports appear suitable for lignin conversion. The highly active and rapidly deactivating zeolite-supported hydrotreating catalyst afforded a complex mixture of products. Regeneration of the alumina-supported catalysts would be required.