A new theoretical development based on the concept that the air-water interfacial layer of adsorbed ionic surfactant constitutes a nonautonomous phase is presented. This adsorbed layer can thus be described by a state function of variables whose values are dependent on the adjacent bulk phase. This thermodynamic analysis allows demonstration that, for a given interfacial amount of adsorbed ionic surfactant, a Corrin-Harkins-like relationship applies when varying the concentration of an inorganic salt having the same counterion in common with the ionic surfactant. Experimental validation of this analysis is done using the data of Tajima, who studied with a radiotracer method the effect of sodium chloride on the adsorption of sodium dodecyl sulfate (Na+DS-) at the air-water interface, The Corrin-Harkin-like relation, Log(a(DS-)) + beta Log(n(Na+)) = K, is verified with excellent correlation coefficients for various amounts of adsorbed DS- at the air-water interface. This result suggests that the behavior of the ionic surfactant layer at the air-water interface is similar to that of micelles. In particular, beta may constitute a scale for quantifying the degree of counterion binding to the interfacial layer of surfactant. Finally, the presented model allows the concept of surface stoichiometry for the calculation of the surface area occupied by the polar headgroup of a ionic surfactant at an air-water interface to be introduced.