Self-consistent periodic slab calculations based on gradient-corrected density-functional theory (DFT-GGA) were conducted to probe the potential-energy diagram for the hydrogenation of propanal and acetone on Pt(111). Calculations for molecularly adsorbed species indicate that acetone and propanal are both weakly bound to the surface through oxygen (i.e., energy changes of adsorption near -20 kJ/mol). The activation energy barriers are calculated to be similar to60 and 40 kJ/mol for the addition of an adsorbed hydrogen atom to adsorbed acetone and propanal, leading to adsorbed isopropoxy and n-propoxy species, respectively. The subsequent hydrogenation steps to form adsorbed alcohol species have activation barriers near 15 kJ/mol. These results would suggest that the rate of propanal hydrogenation over Pt should be faster compared to acetone hydrogenation, in contrast to the behavior observed experimentally [G.M.R. van Druten and V. Ponec, Applied Catalysis A: General 191, 153 (2000)]. The origin for the experimentally observed slower rate of propanal hydrogenation over Pt appears to be related to the formation of strongly adsorbed spectator species formed by removal of the alpha-H atom from adsorbed propanal. The calculated energy change for cleavage of this C-H bond, leading to adsorbed propionyl and adsorbed hydrogen atom, is exothermic by 76 kJ/mol.