In this work, we have explored the validity of the hypotheses on which rest the Hammett's approach to quantify the substituent effect on a reaction center, by applying two DFT energy decomposition schemes. This is performed by studying the change in the total electronic energy, δδE, associated with a proton transfer isodesmic equilibrium. For this reaction, two sets of substituted benzoic acids and their corresponding benzoate anions have been considered. One of these sets contains para- and meta-substitutions, whereas the other one includes ortho-substituted benzoic acids. For each case, the gas phase change in the total electronic energy has been calculated, and two DFT energy decomposition schemes have been applied. The experimental σ X was found to be nearly proportional to the computed δδE. The results for the para- and meta-substituted benzoic acids lead to the conclusion that it is possible to treat separately and, in an additive manner, the electrostatic and steric contributions; and also that the Hammett constant depends mainly on the electronic contributions to the free energy, while the steric contribution is negligible. However, the results for the ortho-substituted cases lead to the conclusion, as was assumed by Hammett, that there are significant qualitative differences between the effects on a reaction site of substituents in the meta- and para-positions and those in the ortho-position.