When selecting ionic liquid (IL) as solvent for extraction processes, the cation alkyl chain length is an important structural variation, while research particularly concerning its effect is lacking. In this work, the effect of cation alkyl chain length on the liquid-liquid equilibria of {IL + thiophene + heptane}, an example for extractive desulfurization, was systematically investigated. The COSMO-RS model was employed to calculate the distributional coefficient (beta(infinity)) and selectivity (S-infinity) at infinite dilution for thiophene of the combinations of 17 cations and 63 anions. Two different anion-dependent tendencies of the beta(infinity) with increasing the cation alkyl chain length were identified and analyzed by COSMO-RS theory. To verify the scarcely-reported decreasing beta(infinity) with longer cation alkyl chain, acetate ([OAc](-)) was selected as representative and liquid-liquid equilibria {[CnMIM][OAc] + thiophene + heptane} (n = 2, 4, 6, 8) were experimentally determined, which validate the anion -dependency and demonstrate the highest extraction performance of [C2MIM][OAc]. (C) 2016 Elsevier B.V. All rights reserved.