The protonation consts. of a series of mono-substituted benzoic acids in aq. soln. in the temp. range 5-55°C has been detd. The dependence of the consts. upon temp. has been analyzed under the light of a statistical thermodn. model assuming the existence in the system of discrete enthalpy levels. Each level is assocd. with one protonated species AHi. The true equil. const. k° is bound to the apparent const. by kapp = k°[W]-nw where [W]nw represents the solvent. The values of nw for monosubstituted benzoic acids either calcd. from the protonation consts. here detd. or from those reported by others range from 1.4 to 2.6 with an av. value nw(av) = 2.1 very close to that obsd. in aliph. acids.