Abstract A surface equation of state for globular proteins at the air-water interface accounting for the structure of the protein molecule, its degree of unfolding, and segment-segment, segment-solvent, and electrostatic interactions is proposed. The proposed lattice model employs the simplifying assumption that all the adsorbed segments are present in the form of trains. Results obtained by fitting the equation of state to the experimental data indicate that the average degree of unfolding of bovine serum albumin (BSA) is greater than that of lysozyme. The number of segments adsorbed per molecule of BSA is found to vary linearly with the surface concentration, whereas the segments of lysozyme adsorbed at the interface are fairly independent of surface concentration. The segment-solvent interactions of the adsorbed segments of BSA and lysozyme are found to be unfavorable (χ > 0.5) due to the exposure of hydrophobic functional groups resulting from unfolding. The maximum surface pressure and surface concentration for monolayer coverage are highest at p I and their predicted variation with pH is in good agreement with the experimental data.