The interactions between milk proteins, beta-Lactoglobulin (beta-Lg) and bovine serum albumin (BSA), at the air-water interface have been evaluated. The surface pressure (pi), molecular area (a) isotherms were obtained by compression of the monolayers at different pH and temperature. In the method used to calculate the interactions, the desorbed segments of the proteins into the aqueous subphase have been considered. Earlier, the desorbed segments have been estimated from the compressibility factor, z, as a function of the surface pressure (virial state equation). The main conclusion from this study is that for biopolymers it is not possible to apply only the mixing thermodynamics to evaluate the intermolecular forces. It is necessary to include the desorption phenomenon. From these results, we can conclude that the main interaction between both proteins is of electrostatic character.