We present a theoretical study of the substituent effects on keto reversible arrow enol equilibria in 10 acetyl derivatives (CH3COX, X = H, OH, CH3, OCH3, NH2, N(CH3)(2), OCHO, F, Cl and Br). The analysis performed in terms of the potential energy, electronic chemical potential, and molecular hardness leads to the following results: (a) in the whole series, the keto isomers are more stable than the enol ones; (b) it is shown that the HSAB principle may explain the relative stability of the keto and the enol species; (c) accurate activation barriers are predicted from a modified Marcus-like equation that contains the force constants associated to reactants and products and to the imaginary frequency of the transition states; (d) the energy and hardness profiles are opposite to each other, verifying the principle of the maximum hardness.