A thermodynamic treatment of protein adsorption layers is presented and an equation of state of the interfacial layer is derived, Only by assuming different models for the activity coefficients in the interfacial layer different are models derived considering ideal and nonideal entropy and enthalpy of mixing, As special cases, well-known equations from other authors are obtained which are frequently used for the description of surfactant and polymer adsorption, Concerning the configuration of adsorbed protein molecules, i.e., number of protein segments adsorbed, the composition of the adsorption layer is caused by a self-regulation process which is mainly influenced by the chain flexibility and determines the surface pressure, Only four basic parameters describe the adsorption equilibrium of the protein, A sufficiently good agreement between literature data and the model is obtained, although best-fit procedures do not exist so far and the isotherm parameters were adjusted only manually.