The surface pressure (pi)-area (A) isotherms and Brewster angle microscopy (BAM) of monoglyceride and whey protein isolate (WPI) mixed films spread on buffered water at pH 5 and 7 and at 20 degreesC were determined as a function of the mass fraction (X) of monoglyceride (monopalmitin or monoolein) in the mixture. From the X and pi -dependence on excess area, free energy, collapse pressure, BAM images, and the evolution with pi of the relative reflectivity (I) of BAM images, it was deduced that the structural characteristics, miscibility, and morphology of monoglyceride-WPI mixed films are very dependent on surface pressure and monolayer composition. The monolayer was more expanded as the monoglyceride concentration in the mixture was increased. Over the overall range of existence of the mixed film, the monolayer presents some heterogeneities. At higher T, after the WPI collapse, characteristic squeezing-out phenomena were observed. At the monoglyceride monolayer collapse, the mixed film was practically dominated by the presence of monoglyceride. However, some degree of interactions exists between monoglyceride and WPI in the mixed film, and these interactions are more pronounced as the monolayer is compressed at the highest surface pressures.