The paper presents new data on the activity coefficient at infinite dilution (gamma(infinity)(13)) for 29 various organic solutes: alkanes, alkenes, al-kynes, cycloalkanes, alcohols, ethers, ketones and aromatic hydrocarbons in two quaternary-ammonium based ionic liquids (quat-based ILs): didecyldimethylammonium perchlorate [DDA][ClO4] and didecyldimethylammonium tetrafluoroborate [DDA][BF4]. The activity coefficient at infinite dilution and physicochemical properties of two mentioned ionic liquids, were determined by inverse gas chromatography at four different temperatures: 308K, 323K, 338K and 353K. Additionally, on the basis of the specific retention volume (V-g(0)) the Flory - Huggins interaction parameters (chi(infinity)) were determined. In order to identify the fundamental interaction between solute and solvent inside the column, the partial molar excess enthalpies at infinite dilution (Delta H-1(E,infinity)) were calculated on the basis of experimental gamma(infinity)(13) values. The selectivity (S-ij(infinity)) and capacity (k(j)(infinity)) at infinite dilution for a few different separation problems: hexane/benzene, cyclohexane/benzene and hexaneitetrahydrofuran were calculated from the experimental gamma(infinity)(13) values to evaluate the possible use of the ionic liquids studied as a new class of solvents in liquid-liquid extraction and in other extraction or microextraction methods. The values of the above-mentioned parameters were compared and discussed taking into regard the chemical structures of both quaternary ammonium-based ionic liquids, including the same cation and their interactions with the organic solute. (C) 2018 Published by Elsevier B.V.