The adsorption of mixed beta -casein/beta -lactoglobulin films to the air/water interface and the subsequent displacement by the nonionic surfactant Tween 20 was studied. A combination of fluorescent labeling of the protein and Langmuir-Blodgett deposition was used to study the mixed protein layer. The adsorption was also monitored using two surface rheological techniques, shear and dilatation. Fluorescent labeling was able to show that to within the limits of optical resolution the two proteins were well mixed at the interface. We also show that the film remained well mixed after 3 days. Surface rheological data from the two techniques used was self-consistent and showed that during the initial stages of development, the films were dominated by the adsorption of the beta -casein. Both fluorescence microscopy and atomic force microscopy were used to follow the displacement of the mixed film by surfactant. Results on films displaced by the nonionic surfactant Tween 20 showed that beta -casein was preferentially displaced from the mixed film before beta -lactoglobulin.