In the following, we focus on two-component solutions composed of a single electrolyte and water. We demonstrate that the separation of the solution into a well-chosen, uniquely defined, probe volume and its environment leads to a generalized multipole description of the excess interaction energy of electrolyte solutions. The number of required expansion orders is small: even for complex electrolytes such as ZnCl 2 and LiCl three components are sufficient to describe the experimental data. When integrated, the resulting equation yields an analytical form of the osmotic coefficient ϕ and, thus, the water activity a w . The set of equations is applied to a total number of 135 electrolytes. The dependency of the fit parameters on electrolyte composition is discussed.