Long-term relaxation phenomena in beta-casein-monoglyceride mixed films at the air-water interface were studied using a surface film balance and Brewster angle microscopy. Relaxation in surface area at a constant surface pressure (at 20 mN/m), just below the beta-casein collapse, or at a constant area at the collapse point of the mixed film has been analyzed according to models for desorption, collapse, and/or organization/reorganization changes. At a constant surface pressure, the organization/reorganization change of beta-casein molecules in monopalmitin- and monoolein-beta-casein mixed films is the mechanism that controls the relaxation process. At the collapse point of the mixed film, the relaxation phenomena may be due either to nucleation and growth of critical nuclei of monoglyceride or to a complex mechanism including competition between desorption and monolayer collapse. For beta-casein-monolaurin mixed films, the relaxation phenomena are mainly due to the irreversible loss of monolaurin molecules by desorption into the bulk aqueous phase.