This research work aims to investigate the behavior of a mixed system composed of a commercial protein (whole casein) and a low molecular weight surfactant (Tween 20) in order to understand its foam stability on the basis of fundamental surface quantities such as surface and disjoining pressure. These experiments prove to be extremely useful in the understanding of the processes determining foam stability. The complex behavior of whole casein/Tween 20 mixtures is directly deduced from the surface pressure isotherms. Concretely, the isotherm of the mixed system is displaced to smaller surfactant concentrations as compared to the pure surfactant system. This feature is quantified by a critical aggregation concentration and suggests formation of protein/surfactant complexes within the surface layer. The disjoining pressure isotherms of the pure and mixed systems provide key information regarding the structure and composition of the mixed whole casein/Tween 20 surface layers. Furthermore, they provide a direct correlation with the foam stability in terms of the thickness of the final foam film; the thinner the film, the less stable the foam. The experimental results are further discussed in terms of literature studies of similar systems, and a final rather accurate description of the system arises. This work investigates the importance of the nature of the protein in the stability of the foams of protein/surfactant mixtures and highlights the fundamental role of the surface properties in the understanding of such a major phenomenon in colloid science and technology.