On the basis of quasiton theory the equations of state were obtained for a two-component interface separating liquid and gas phases with a surface-active additive to the condensed matter. The first equation describes the state of a surface layer of a binary liquid, first for a liquid metal. The second equation is that of a surface layer of a vapor in contact with the condensate. With the help of these equations computations were conducted for the following systems: Fe-O, Fe-N, Fe-S, Hg-Na, Hg-K, Hg-Cs, He-4-He-3. Except in the last case, the thickness l(1) of the surface layer of liquid is close to the atomic radii of the additives, for the most part, but the thickness l(2) of the surface layer of the gas is sometimes of an order of magnitude greater than l(1).