A detailed thermochemical analysis of the α-cleavage and decarbonylation reactions of acetoneand several ketodiesters was carried out with the B3LYP/6-31G* density functional method. Theheats of formation of several ground-state ketones and radicals were calculated at 298 K to determinebond dissociation energies (BDE) and radical stabilization energies (RSE) as a function ofsubstituents. Results show that the radical-stabilizing abilities of the ketone substituents play avery important role on the thermodynamics of the α-cleavage and decarbonylation steps. An excellentcorrelation between calculated values and previous experimental observations suggests thatphotochemical α-cleavage and decarbonylation in crystals should be predictable from knowledgeof excitation energies and the RSE of the substituent.