The adsorption and average activity coefficient of sodium dodecyl sulfate and the adsorption of sodium halide in sodium dodecyl sulfate-sodium fluoride (chloride, bromide)-water systems were calculated based on the experimental values of the modulus of elasticity of films and the surface tension of solutions using previously derived equations. Premicellar aggregation of surface-active ions takes place in the solutions studied in concentrations 10-20 times below the critical micelle concentration, and this directly affects the value of the activity coefficient and requires its obligatory consideration in calculations with the adsorption equation.