In this work we describe a simple thermodynamic method for determination of the adsorption (amount per unit area) of ionic surfactant. The latter is obtained from the interfacial tension isotherm measured in the presence of arbitrary (and fixed) concentration of inorganic electrolyte. Polynomial fit of the interfacial tension versus the logarithm of the mean ionic activity is combined with the Gibbs adsorption equation written in a form suitable for arbitrary content of salt. This procedure is an extension of the approach of Rehfeld [J. Phys. Chem. 71 (1967) 738]. We have performed measurements with sodium dodecyl sulfate (SDS) on water/air and water/oil (n-hexadecane and Soybean Oil) boundaries, at different salt concentrations (10 and 150 mM NaCl). Wilhelmy plate method was used for the water/air measurements; for water/oil we applied drop shape analysis with pendant drops. The obtained isotherms, together with literature data, are processed and the adsorption is determined. The results are compared and discussed in view of the role of the salt and the type of the hydrophobic phase. On oil/water boundaries the adsorption is always lower than that on air/water surface; addition of inert electrolyte increases the adsorption. We analyze theoretically the asymptotic behavior of the adsorption as a function of the solute concentrations in the limit of high surface coverage. The treatment is based on models existing in the literature (Langmuir isotherm with account for the counterion binding, as formulated by Kalinin and Radke [Colloids Surf. A 114 (1996) 337]; the activity coefficients were taken into consideration in the frames of the Debye-Hückel theory). The obtained asymptotic functional dependence of the adsorption is used for fitting of data. The agreement is always good, in the concentration region below and near the critical micellization concentration (CMC). From the fits we determine the limiting adsorption at maximum coverage (i.e., at saturation); therefrom, the degree of coverage of the interface with surfactant is estimated. It turns out that at the CMC the coverage is lower than about 90%. Thus, we confirm literature results for absence of saturation with ionic surfactants at the CMC. The dependence of the surface coverage upon the mean ionic activity is rather insensitive toward the type of the fluid interface (air/water, oil/water with different hydrocarbons), and the salt concentration.