An ideal associated gas film model is proposed for highly expanded insoluble nonionic surface films, based on a single equilibrium constant for stepwise addition to form aggregates ranging from two molecules to infinity. This model yields a single-parameter equation of state, which appears to be superior to the van der Waals equation for describing the properties of the surface film from near the point of “liquid vapor” equilibrium to infinite expansion. This equation provides a simple method for calculating the thermodynamic properties of gaseous and condensed films, and is particularly useful for estimating the mixing properties of films. © 1982.