A thermodynamic description of condensation of droplets on ions is presented. The Gibbs free energy of noninteracting droplets in equilibrium with their vapor is a function of the number of moles, temperature, pressure, and surface tension G (p(r), Tau, sigma, n(1)(g),...,n(c)(g); n(1)(1),... n(c)(1); n(c)(1); n(1)(sigma),...n(c)(sigma)). From the variation around equilibrium of G, an equation relating saturation pressure and droplet size is obtained. The saturation ratio is considerably reduced by adding an ionic salt (the solute). Ion hydration allows the formation of stable droplets in an unsaturated environment. Their effect prevails at small values of the droplet radius r where the equation exhibits a stable branch and predicts condensation, even in the case of a perfect ionic solution. The equation also shows that an ionic mixture behaves as a pure solvent (metastable branch, Kelvin limit) for large r. We compare our results with those previously obtained.