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, gamma(infinity)(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(8)iQuin][NTf2], 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 Delta H-1(E,infinity) were calculated from the experimental gamma(infinity)(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 gamma(infinity)(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. (C) 2010 Elsevier Ltd. All rights reserved.