Equilibrium surface tension isotherms of sodium dodecylbenzene sulfonate (DDBS) are obtained at various fixed concentrations of NaCl. The contents of unsulfonated dodecylbenzene (DDB) in the used surfactant sample is determined by processing the surface-tension data. Having determined the parameters of the best fit, we computed the adsorption of surfactants (anionic DDBS and nonionic DDB), the binding of counterions in the Stern layer, the surface electric potential, the surface elasticity, etc., each of them for various surfactant and salt concentrations. The results show that for the solutions without added NaCl, the adsorption layer consists mostly of the nonionic DDB, irrespective of its small mole fraction in the surfactant blend. The admixture of DDB in the sample of DDBS leads to a significant increase of the surface elasticity. Moreover, even minimal added amounts of CrCl3 or Fe-2(SO4)(3) cause a considerable reduction in the surface tension, which is due to the greater binding energies of some of the counterions released by the latter electrolytes. The paper gives a quantitative analysis and description of the adsorption from aqueous solutions of a technical ionic surfactant. The followed strategy, which was to determine the contents of the admixtures and to account for their presence in the theoretical model, rather than to purify the surfactant, may find applications to other mixed surfactant systems.