The energies of acetic acid and its CH3C(=O)X derivatives and of the corresponding enols H2C=C(OH)X, for X = OH, NH2, NMe2, OMe, OCHO, F, Cl, and Br, were calculated by the MO ab initio method MP2(full)/6-31G**, by single-point CCSD(T)(full)/6-311G**/MP2(full)/6-31G**), and by the hybrid density functional method B3LYP/6-31G**. The calculated pK(Enol) = -log K-Enol values for the "keto"/enol equilibria are all high (17.5-24.0) and follow the order for X:H < alkyl < OCHO < Br similar to Cl < F < NH2 < NMe2 < OH, OMe. By using the appropriate isodesmic reactions, it was shown that all substituents stabilize more the acid species than its enol (where Br and Cl are slightly destabilizing). All the computational methods display a similar bend. The structures and energies of the various conformations of both species are given. Comparison with the scarce experimental pK(Enol) data shows a reasonable agreement with the calculations.