Four ionic liquids (ILs) consisting of the bis(trifluoromethylsulfonyl)imide anion paired with a phosphonium, pyridinium, imidazolium, or thiolanium cation were investigated as potential solvents to separate aromatic from aliphatic compounds in a liquid-liquid extraction process. The thiolanium IL was chosen due to its structural similarity to sulfolane, which is the most widely used organic solvent for aromatic/aliphatic separation in the industry. Interestingly, ternary liquid-liquid equilibrium data shows that 1-n-butylthiolanium bis(trifluoromethylsulfonyl)imide performs as well as the equivalent imidazolium IL despite the fact that it is not aromatic and cannot use 1-pi interactions (which are available to imidazolium ILs) to enhance aromatic solubility and selectivity. Moreover, we provide quantification of the IL solubility in the organic-rich phase, which is on the order of pi-pi mole fraction. This quantity is important 10(-4) because it would represent IL loss and product contamination in a real extraction process; however, it is commonly reported to be nondetectable. The nonrandom two-liquid (NRTL), perturbed-chain statistical associating fluid theory (PC-SAFT), COSMO-RS, and COSMO-SAC models are appropriate for the mixtures explored, and each model's strengths and weaknesses are discussed.