COSMO-RS is a widely accepted method to calculate thermodynamic properties like partition coefficients. VLE-, and LLE-data. At present COSMO-RS cannot account for long-range ion ion interactions. Further. the element specific COSMO-radii for cations like lithium or potassium have not been optimized. In this work an extension of COSMO-RS for monoatomic electrolytes is presented. Based on experimental mean ionic activity coefficients in aqueous solutions new element specific COSMO-radii for alkali metals were introduced. The new parameterization accounts for long-range ion ion interactions and cation hydration as well as an element specific hydrogen bonding contribution for anions. In order to evaluate the new parameterization, mean ionic activity coefficients in mixed solvent systems were calculated and compared to experimental data. In addition, liquid liquid equilibria in presence of electrolytes were calculated with the new parameterization. The influence of monoatomic electrolytes on immiscible aqueous organic systems can be accurately reproduced. Further, the salt induced phase separation of miscible aqueous organic system was successfully predicted. (c) 2011 Elsevier B.V. All rights reserved.