A novel surface equation of state obtained from the BET adsorption isotherm and the Gibbs equation is presented. This equation describes both monolayer and multilayer adsorption. The emphasis of this paper is on monolayers, for which this new equation only contains thermodynamic parameters. In this form, it describes the characteristic behavior of nonionic surfactants, the relation between maximum surface pressure and critical micelle concentration, and the link between the standard free energy of adsorption and the standard free energy of micellization. It also explains and justifies the fundamental work of Rosen regarding the relative effects of structural factors on the surface tension reduction. The deviation of the model from experimental data is explained in terms of the activity coefficient at the critical micelle concentration, which is proportional to the micelle aggregation molecular weight.