Activity coefficients at infinite dilution have been measured with the dilutor technique for 20 solutes (alkanes, alkenes, cyclic hydrocarbons, aromatic hydrocarbons, ketones, alcohols, and water) in the ionic liquids 1-methyl-3-methyhmidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium bis(txifluoromethylsulfonyl)imide, and 1-ethyl-3-methylimidazolium. ethylsulfate. The measurements were carried out in the temperature range between 293.15 K and 333.15 K. The selectivities at infinite dilution (S-ij(infinity) = gamma(i)(infinity)-/gamma(j)(infinity)) for the separation of aliphatics from aromatics and n-hexane from 1-hexene are presented and discussed. From the results it can be concluded that the ionic liquids investigated show different advantages compared to those of the entrainers actually used for the separation of aliphatic from aromatic hydrocarbons by extractive distillation or extraction.