Thermodynamic equations for describing protein adsorption layers at liquid/fluid interfaces are derived as a generalization of a theory published recently [J. Colloid Interface Sci. 1996, 183, 26]. In this new theory the nonideality of enthalpy (Flory-Huggins' parameter) and entropy of mixing are taken into account, and also the effect of the electric charge of the protein molecules on surface pressure is considered. The model is verified by experimental dynamic and equilibrium surface tension data for HA solutions obtained from pendent drop experiments (ADSA), The derived isotherm is in good agreement with the experimental data. The values of the isotherm parameters surface area per molecule, electric charge of the HSA molecule, and the adsorption layer thickness are close to values obtained by other methods in literature. It follows that HSA molecules undergo almost no denaturation at the solution/air interface and occupy a surface area of about 50 nm(2), independent of the packing in the adsorption layer, which is in agreement with the concept of a triple-domain structure.