A high-pressure once-through plug-flow-reactor system is reported for characterization of hydroprocessing reactions of biomass-derived compounds. All of the reactants are liquids. Data are presented for conversion of guaiacol catalyzed by Pt/MgO and, in much less detail, CoMo/Al2O3. The data demonstrate the advantages of high pressure in such reactions, determining a pseudo-first-order rate constant for guaiacol conversion at 523 K and 69 bar of approximately 55 L of organic reactant solution (g of catalyst)(-1) h(-1). This value is 2 orders of magnitude greater than that observed at 573 K and 1.4 bar.