The thermodynamic model of Fainerman and Miller (Langmuir 1996, 12, 6011) for the description of ideal adsorption layers of surfactants forming small surface aggregates is generalized for the case of very large aggregates in the surface layer, for nonideal surface layers, and ionic surfactants. The models are used to explain results in the literature. It is shown that the transition region observed in SDS adsorption layers on concentrated NaCl substrates as discussed for example by Nikolov, Martynov, and Exerowa (J. Colloid Interface Sci. 1981, 81, 116) is due to the formation of surface aggregates with an aggregation number of the order of 100.