Reaction enthalpies for the addition of hydride ion to carbonyl compounds are calculated at the G2(MP2) level of theory for 34 compounds including carbonyl compounds XYCO (where X, Y = H, CH3, NH2, OH, F), alpha,beta-unsaturated aldehydes, cycloalkanones, formaldehyde complexed with Lewis acids, and heterocarbonyl compounds. Hydride affinities are analyzed in terms of two isodesmic reactions. These reactions fit available experimental data extremely well, in most cases within 1 kcal/mol. It is concluded that relative to hydrogen, electron withdrawing substituents stabilize both alkoxides and carbonyls, but stabilize alkoxides more. Interaction with a lone pair on a substituent also stabilizes carbonyls, though alkoxides are unaffected. Hydride affinity is most exothermic when inductive effects are largest and lone pair interactions are minimized. Hydride affinities are strongly correlated to the atomic charges of the carbonyl carbon and oxygen but are independent of geometry. The addition of methide, amide, hydroxide, and fluoride to formyl derivatives is examined. Trends in reaction enthalpies relative to addition to formaldehyde are independent of the identity of the nucleophile.