On the basis of our previous experimental findings as well as highly accurate activity data referring to dilute 1-propanol-water solutions at 298 K, it was possible to calculate the chemical potentials of solution components as a function of sorbent concentration. Combining the thermodynamic data with those obtained from mass-balance equations and assuming applicability of the zeroth law of thermodynamics for the interfacial layer, the average surface activity coefficients can be calculated. It was pointed out theoretically that a certain, non-Gibbsian type of excess can be given purely from an appropriate thermodynamic treatment of the experimental data. It has also been proven that only the so-called characteristic isotherm of the adsorption system can be directly related to thermodynamic equations. At finite sorbent concentrations, however, only a kind of combination of thermodynamic and mass-balance methods can give some characteristic parameters of the sorption layer. An equation is proposed to account for the free enthalpy change due to the difference in concentration between the sorption layer and the bulk solution.