The partition coefficients of benzothiophene (BT) and benzothiophene 1,1-dioxide (BTDO) in the octane/acetonitrile system were predicted using the COSMO (conductor-like screening model) based activity coefficient models COSMO-SAC (segment activity coefficient) and COSMO-UNIQUAC. In COSMO-UNIQUAC, a segment activity coefficient was described in a UNIQUAC-type equation. The results with COSMO-UNIQUAC were compared with those from the COSMO-SAC model. It was found that COSMO-UNIQUAC reproduced the experimental data more accurately than COSMO-SAC. The surface charge density profiles (sigma-profiles) of functional groups on BT and BTDO molecules were utilized to determine that the partition coefficients of BTDO are much lower than those of BT. A hypothetical molecule with the volume and surface area of BTDO was conducted by using the a-profiles of functional groups on BT and BTDO. The partition coefficients of the hypothetical component were predicted by COSMO-UNIQUAC. It was revealed that the partitions of BTDO in the octane/acetonitrile system were influenced from the charge density changes by the oxidation on not only the sulfur atom but also on the carbon and hydrogen atoms.