Bio-oil derived by fast pyrolysis of biomass represents a potentially attractive source of hydrocarbon transportation fuels. Raw bio-oil, however, is unsuitable for application as a fuel due primarily to high organic oxygen content, which imparts a number of undesirable properties including high acidity and low stability. These problems can be overcome by catalytic hydrodeoxygenation; however, removing oxygen to very low levels by hydrotreating carries a strong economic penalty. Mild hydrotreating (where moderate levels of deoxygenation take place) coupled with coprocessing in a petroleum refinery represents an alternative to deep hydrotreating which may improve the economics of manufacture of hydrocarbon transportation fuels from biomass. This study reports on the effect of catalyst type on the quality of bio-oil produced via mild hydrotreating in a semibatch reactor at three severities. Sulfided Ni-Mo/Al2O3, Pd/C(activated), Pd/char, Pt/char, and Ru/char were compared. Speciation of oxygen functional groups in distillate and bottom products was carried out, and the form of much of the organic oxygen was determined. These results show that a 55% conversion of the carbon in the biomass pyrolysis oil to a low-oxygen (5%), low-acid, volatile, hydrocarbon-miscible liquid product can be achieved. This was, however, possible only with the NiMo-S catalyst. The precious-metal catalysts while producing oil with acceptable carbon conversion, miscibility, and oxygen content did not convert enough acid to produce oil with acid numbers below 15. Water washing was successfully tested for removing residual acids. The various catalysts have different advantages, and using a different catalyst for the two stages of the process may provide the best process-for example ruthenium for minimizing coke during stabilization and nickel or platinum for deoxygenation.