The simultaneous interaction of thiophene and pyridine with different ionic liquids: 1-butyl-1-methylpyrrolidinium tetrafluoroborate([BPYRO][BF4]), 1-butyl-1-methylpyrrolidinium hexafluoro-phosphate ([BPYRO][PF6]), 1-butyl-4-methylpyridinium tetrafluoroborate ([BPY][BF4]), 1-butyl-4-methylpyridinium hexafluorophosphate ([BPY][PF6]) and 1-benzyl-3-methylimidazolium tetrafluoroborate ([BeMIM][BF4]) were investigated using quantum chemical calculations. A three-tier approach comprising of partial charges, interaction energies and sigma profile generation using conductor-like screening model for real solvents (COSMO-RS) was chosen to study the systems. A quantitative attempt based on the CH-p interaction in ionic liquid; thiophene-pyridine complexes gave the interaction energies of ILs in the order: [BPY][BF4] > [BPYRO][PF6] > [BeMIM][BF4] > [BPY][PF6] > [BPYRO][BF4]. An inverse relation was observed between the activity coefficient at infinite dilution predicted via COSMO-RS-based model and interaction energies. The dominance of CH-p interaction was evident from the sigma profiles of ionic liquid together with thiophene and pyridine. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 749-764, 2011