For the first time the a-priori prediction method COSMO-RS is used to predict liquid chromatographic partitioning coefficients. Previous methods are based on several adjustable parameters used to describe physico-chemical properties of the mobile and stationary phase in the separation system examined. Due to the empirical parameter fitting, predictivity of these methods is limited. The COSMO-RS approach replaces adjustable parameters by quantum chemistry. Furthermore the complete thermodynamic information of the separation system is known. An empirical method to improve the results is shown. Its validation comprises various eluent systems lt is demonstrated that an a-priori choice of appropriate phases for chromatographic separation systems is possible. Furthermore it is a tool to guide chemists how to synthesize new stationary phases.