The study of mixed films has become of consider able interest both from theoretical and practical points of view. The pi-A isotherms of monostearin-distearin (about 50% 1,3- and 50% 1,2-isomers) mixed films spread on water as a function of emulsifier ratio and temperature were studied in an automated Langmuir-type film balance. When molecules of both emulsifiers are spread on the air-water interface they are packed more closely together than when either emulsifier is present alone, and they enter into some form of association This association can be demonstrated by surface pressure-area isotherms and by thermodynamic studies. The characteristics of the mixed films, their elasticity and miscibility, which are of interest in emulsion ol foam stability are discussed. A molecular model established can explain the interactions between film and subphase molecules and their relationships with variables of the process (pressure and temperature). This model fits the calculated thermodynamic parameters.