On the basis of the thermodynamics of the stoichiometric displacement model for adsorption of a solute, two equations to calculate separately two fractions of the Gibbs free energy for the solute adsorption and the solvent desorption to be the functions of temperature and the solute activity in bulk solution, were deduced. It was found that Delta G((beta a)) (the affinity Gibbs free energy change between an adsorbate and adsorbent) in any equilibrium activity of the solute, alpha(PDm),, and another fraction Delta G((q/Z,PDm)) (the desorption Gibbs free energy of a solvent from an adsorbent) under the same alpha(PDm), are linear functions of temperature. When temperature is fixed, Delta G((beta a)) is a constant while Delta G((q/Z,PDm)) is proportional to log alpha(PDm). The changes in enthalpy and entropy corresponding to the solute adsorption and the solvent desorption can also be derived from the plot of the foregoing calculations of Delta G((beta a)) and Delta G((q/Z,PDm)) vs. thermodynamic temperatures T, respectively. Experimental data on isotherms from the literature are used to test all of the fractions of thermodynamic function changes computed quantitatively with the equations presented in this study, and a satisfactory result was obtained.