Relaxation phenomena in a whey protein isolate (WPI) and monoglycerides (monopalmitin, monolaurin, and monoolein), mixed films at the air-water interface were studied using surface film balance and Brewster angle microscopy (BAM). Relaxation in surface area at a constant surface pressure (at 20 mN/m) or at constant area at the collapse point have been analyzed according to models for, desorption, collapse, and/or organization/reorganization changes. At a constant surface pressure, the organization/reorganization changes of WPI molecules in mono-palmitin- and monoolein-WPI 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 WPI-monolaurin mixed films the relaxation phenomena are mainly due to the irreversible loss of monolaurin molecules by desorption into the bulk aqueous phase.