A thermodynamic equation of the surface tension isotherm for systems with an arbitrary number of electrolytes has been derived from the Gibbs adsorption equation with the use of mean rational activity coefficients. The application of the equation obtained to strong and weak electrolytes has been discussed. It is shown that the equation that is derived is equally applicable to strong and weak electrolytes and makes it possible to perform calculations of the extent of adsorption regardless of the form in which each electrolyte exists on the surface and in the bulk phase.