The Hammett equation has for the first time been applied to the esterification in the gas phase of m-MeO, p-MeO, m-Me, p-Me, H, m-NO2, m-Cl, and m-Br benzoic acids, catalyzed by AlPO4 of the F type. It is shown that the Hammett equation could be applied to the adsorption equilibrium constant, KA, and to the apparent kinetic constant, [Formula: see text] The KA and [Formula: see text] values used in this paper have been taken from a study by the same authors. The m-NO2 benzoic acid is in disagreement with the general behavior of the other acids in the application of the Hammett equation to the adsorption process. The p-MeO benzoic acid is in disagreement with the behavior of the acids in the application of the equation to the surface reaction. In these cases, the ρ values were negative. It can be deduced that a positive charge is generated in the adsorption process and in the surface reaction. The AAc2 mechanism is discussed according to the values of ρ, and a more detailed mechanism is proposed. The separation of the enthalpic and entropic contributions to the substituent effect is carried out. [Formula: see text] and[Formula: see text] are nearly 50% in the adsorption process. In the surface reaction, the entropie contribution is greater (72.5%) than the enthalpic (27.5%).