We report the X-ray and neutron reflectometry measurements of the structural changes caused by chemical denaturation of a surface excess of the bovine milk protein, beta-lactoglobulin. The thickness of the diffuse protein surface layer was used as an order parameter as there was no corresponding increase in the surface excess as a function of guanidinium chloride (G.HCl) concentration. A thermodynamic analysis performed gave the interfacial free energy of unfolding in the absence of a denaturant (Delta G(0)). This energy, lower than the free energy of unfolding bulk solution, shows that the air-water interface has a destabilizing effect on protein structure up to 50 kJ mol(-1).