The lipophilicity of a compound is generally described by its n-octanol/water partition coefficient. Lipophilicity is an important descriptor, e.g. to correlate the toxic effect of substances or adsorption and transport properties of drug molecules in the human body. Often drug molecules contain one or more ionizable groups, hence the lipophilicity is strongly pH-dependent. The thermodynamic model COSMO-RS is a widely accepted method to predict partition coefficients of neutral compounds between neutral solvents and ionic liquids, thus COSMO-RS is principally able to handle molecules containing ionizable groups. In this work the model COSMO-RS was used to predict lipophilicity profiles of ionizable pharmaceuticals containing one acidic group. Under consideration of ion paring the partition of ionizable drugs can be calculated. The predicted pH dependent lipophilicities are in good agreement with literature data and own measurements. (C) 2011 Elsevier B.V. All rights reserved.