The COSMO-RS method, a quantum-chemically based method for the prediction of thermodynamic properties of pure and mixed fluids, has originally been developed and parameterized only for neutral compounds. Nevertheless, without any technically changes toward ions, it has been applied to the prediction of thermodynamic properties of ionic systems, as ionic liquids, dissociation into ions, i.e. pKa, Gibbs energies of transfer of ions from gas phase to water and between aqueous and organic phases, and to the prediction of ionic strength on activity coefficients of solutes in aqueous solution. In some of the cases COSMO-RS was extremely successful, but nevertheless in other situations there are severe limitations. In this talk an overview of the different ion-related applications of COSMO-RS will be given, and rules for the applicability and current limitations of COSMO-RS will be worked out.