The octanol-water partition coefficient (K-ow) of ionic liquids (lLs) is an important indicator for its toxicity and environment impact. In this work, the K., is determined from the ratio of infinite dilution activity coefficient of IL in the water-rich and octanol-rich phases. In particular, the Pitzer-Debye-Huckel (PDH) model combined with the predictive COSMO-SAC model is used for calculating the activity coefficients. A root-mean square deviation of 0.75 is achieved for log K-ow or a factor of 4 in K-ow., for a total of 67 ILs at ambient condition. The long-range coulomb interactions (from the DH model) contribute an almost constant value of -1.35 to log K-ow, regardless of the type of IL The change of log K-ow with the molecular structure of IL is found to be dominated by the short-range attractive interactions between the IL and the solvents, including the hydrogen bonding and nonhydrogen bonding surface interactions. The combination of PDH and COSMO-SAC models provides not only the quantitative predictions of K-ow of ILs and but also physical insights to the relations between K-ow and the molecular structure of ILs. (C) 2013 Elsevier B.V. All rights reserved.