Theoretical calculations on the fluoroacetones and their conjugate acids exhibit an inverse dependence of the proton affinity on the number of fluorine substituents. The good correlation between increasing proton affinity and decreasing ionization potential is attributed to the observation that the ionization of a nonbonding electron is essentially localized at a single atom whieh is also the site of protonation. The best quantitative agreement between the theoretical and experimental proton affinities is obtained at the 6-31G* level, while the STO-3G basis set leads to a satisfactory account of the additivity of fluorine substituents. The calculations corroborate the experimental interpretation of the preferred conformations ofp-fluoroacetone molecules in terms of the presence of an intramolecular hydrogen bond between the carbonyl proton and the F on the adjacent carbon. Only one type of equilibrium conformation is predicted for the fluorinated acetones, whereas two types of equilibrium conformations are predicted for the conjugate bases.