Our theoretical considerations are based on the popular triple-layer model of the surface phase and on assuming a 2-pK charging mechanism. The mechanism, developed by Sverjensky and Sahai with dependence on the pK(a1)(int) and pK(a2)(int) equilibrium constants on the point of zero charge (PZC) of oxides, is combined here with some experimental findings to study the effect:of PZC on the enthalpic effects accompanying ion adsorption. A linear dependence on PZC of the enthalpy changes accompanying the first and the second proton adsorption has been found. The interrelations found in this paper seem to deserve further experimental and theoretical study.