COSMO-based activity coefficient models such as COSMO-SAC (conductor-like screening model-segment activity coefficient) have been shown to be relatively successful predictive models for molecular systems. [As cited in Lin, S. T.; Sandler, S. I. Ind. Eng. Chem. Res. 2002, 41, 899-913 and Mullins, E. et al. Ind. Eng. Chem. Res. 2006, 45, 4389-4415.] In this study, we present an extension of COSMO-SAC to electrolytes (eCOSMO-SAC) that combines the COSMO-SAC term for short-range molecule-molecule, molecule-ion, and ion-ion interactions with the extended symmetric Pitzer-Debye-Huäckel term for long-range ion-ion interactions. [As cited in Song, Y.; Chen, C.-C. Ind. Eng. Chem. Res. 2009, 48, 7788-7797.] The extension recognizes that like-ion repulsion and local electroneutrality govern the surface segment contacts, and it introduces a dual sigma profile concept for electrolyte systems. [Chen, C.-C. et al. AIChE J. 1982, 28, 588-596.] While the model formulation and parameters remain to be optimized with a greater selection of electrolytes, the eCOSMO-SAC predictions for a few representative electrolyte systems show trends that are in qualitative agreement with experimental data and those generated from the eNRTL model (as mentioned in the Song et al. work previously noted) and demonstrate essential characteristics that are consistent with the general behavior of electrolyte systems.