The catalytic hydrodeoxygenation of C3 alcohols (1- and 2-propanol, 1,2- and 1,3-propanediol, and glycerol) on Pt/Al2O3 has been mechanistically explored in the aqueous phase. Dehydrogenation on Pt and dehydration on alumina are the main elementary reaction pathways. In water, carbon–carbon bond cleavage for alcohols with terminal hydroxyl groups occurs via decarbonylation of aldehydes (generated by dehydrogenation of alcohols) and decarboxylation of acids, the latter being formed by disproportionation from aldehydes. The presence of water as solvent suppresses the dehydration for mono-alcohols mainly via blocking of Lewis acid sites by water. Dehydration is still the dominating primary reaction for 1,3-propanediol and glycerol, as the higher number of hydroxyl groups weakens the C–O bond strength. The overall reactivity of C3 alcohols decreases in the order of 1,3-propanediol≈glycerol>1,2-propanediol≈1-propanol.