New isoquinolinium ionic liquid has been synthesized from N-octylisoquinolinium bromide as a substrate. The specific basic characterization of the new compound by NMR spectra, elementary analysis and water content is presented. The activity coefficients at infinite dilution, γ 13 ∞ for 37 solutes viz. alkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons, alcohols, ethers, ketones, and water in the ionic liquid N-octylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide [C 8iQuin][NTf 2], were determined by gas–liquid chromatography at the temperatures from (328.15 to 368.15) K. The partial molar excess enthalpies at infinite dilution values Δ H 1 E , ∞ were calculated from the experimental γ 13 ∞ , obtained over the temperature range. The selectivity for the n-hexane/benzene, cyclohexane/benzene and n-heptane/thiophene separation problems were calculated from the γ 13 ∞ and compared to the other ionic liquids with bis{(trifluoromethyl)sulfonyl}imide anion, NMP and sulfolane, taken from the recent literature. The chosen ionic liquid demonstrates that it is possible to separate different organic compounds with the average selectivity and capacity.