Biomass-derived carboxylic acids frequently contain functional groups such as C=O, OH and furan ring, and the selective removal with retention of carboxyl groups is a useful reaction in biomass valorization. This chapter summarizes the catalytic reduction of such oxygen-containing functional groups in biomass-derived carboxylic acids with H2. Reduction of C=O in levulinic acid is relatively easy with simple metal catalysts to γ-valerolactone by hydrogenation and cyclization and with metal + acid catalysts to valeric acid by hydrodeoxygenation. However, these approaches cannot be applied to wide range of substrates. The use of hydrogen halides as catalysts or co-catalysts with metal is a more general approach, applicable to hydrodeoxygenation of lactic acid, furancarboxylic acids and sugar acids to the corresponding simple acids and diacids, although the corrosive nature of hydrogen halides is a problem. Specific catalysts have been reported for each type of substrates, such as Mo catalysts for lactic acid, Pt-MoOx/TiO2 catalyst for furancarboxylic acids, and noble metal + Re catalysts for sugar acids.