The diverse reactivity of 5-hydroxymethylfural (HMF) in Pd/C-catalyzed reactions is described with emphasis on the role of additives that affect selectivity. Three broad reactions are examined: decarbonylation, hydrogenation, and hydrogenolysis. Especially striking are the multiple roles of formic acid in hydrogenolysis/hydrogenation and in suppressing decarbonylation, as illustrated by the conversion of HMF to DMF. Hydrogenation of the furan ring is suppressed by CO₂ and carboxylic acids. These results emphasize the utility of Pd/C as a convenient catalyst for upgradation of cellulosic biomass.