Molecular thermodynamic approaches to the formation and growth of micelles in aqueous surfactant solutions have proved extremely accurate and useful. The description and prediction of behavior in planar adsorbed layers have not been as thoroughly treated but are clearly of importance to many commercial and industrial surfactant applications. These issues are addressed using the same fundamental free energy components as used in the micelle case, but modified for planar geometry and the presence of an interface between two bulk phases (initially air/water). The explicit consideration of the hydration of adsorbed layers is discussed and is shown to be critical in the correct description of single adsorbed species as well as mixed systems. Comparison is made between the predictions of the model for surface tension and adsorbed area and experimental observations at the air/solution interface for single species, binary mixtures, and the general multicomponent case.