Bifunctional hydrodeoxygenation catalysts containing both metal and metal oxide phases are widely employed in biomass upgrading reactions. Determining the oxidation state of metals in such complex reaction media has been challenging. In this work, we developed a high-pressure trickle-bed reactor capable of conducting temperature programed reduction of catalysts after liquid-phase reactions without exposing the catalyst bed to ambient conditions. Two case studies on metal/metal oxide catalysts employed in key biomass upgrading processes were investigated. The reduction of the RuOx phase in Ru/RuOx/SiO2 occurs at temperatures as low as 115 degrees C via catalytic transfer hydrogenation reactions using liquid 2-propanol as a hydrogen source. Pretreatment of Ir-ReOx/SiO2 catalyst with H-2 in liquid cyclohexane or liquid water reduces Re to an oxidation state of +2.6, while the residual ReOx phase cannot be reduced in H-2 at up to 900 degrees C.