In this research we studied the behavior of mixed monostearin and distearin molecules spread as monolayers at the air-water interface as a function of temperature and surface pressure. The subphase was an aqueous ethanol solution at 0.5 mol/L. The structural characteristics of the mixed films at interface were deduced from pi-A isotherms established using an automated Langmuir-type-film balance. The study has confirmed that mixed films are homogeneous but the mixing process-is not ideal due to the existence of interactions between the components at interface. As a consequence of these interactions, a contraction in the monolayer structure appears. The magnitude of interactions between lipids and ethanol at interface depends on the surface-pressure, the temperature, and the surface composition. The thermodynamic parameters applied to mixed films confirm the existence of these interactions.