The surface pressure (pi)-area (A) isotherms and Brewster angle microscopy (BAM) images of monoolein-beta-casein mixed films spread on buffered water at pH 5 and 7 and at 20 degrees C were determined as a function of the mass fraction of monoolein in the mixture (X). The structural characteristics, miscibility, and morphology of monoolein-beta-casein mixed films were very dependent on surface pressure and monolayer composition. The structure in monoolein-beta-casein mixed monolayers was liquid-expanded-like, as for pure components. The monolayer structure was more expanded as the pH and the monoolein concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area, elasticity, and collapse pressure it was deduced that monoolein and beta-casein form a practically immiscible monolayer at the air-water interface. The BAM images and the evolution with the surface pressure of the relative reflectivity of BAM images give complementary information on the interactions and structural characteristics of monoolein-beta-casein mixed monolayers, which corroborated the conclusions derived from the pi-A isotherm. The morphology of monoolein, beta-casein, and monoolein-beta-casein domains at surface pressures lower than that for beta-casein collapse cannot be observed by BAM due to the fact that pure components and mixed monolayers form isotropic domains at the air-water interface. However, the high relative reflectivity of beta-casein domains after the collapse point leads to the conclusion that monoolein was unable to displace totally the protein from the mixed monolayer at the air-water interface, even at higher monoolein concentrations in the mixture and at higher surface pressures.