The density, viscosity and absorption of ethane and ethylene were determined experimentally, as a function of temperature and at atmospheric pressure, in different imidazolium-based ionic liquids: 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)imide, [C1C8Im][NTf2], 1-methyl-3-(propyn-3-yl)imidazolium bis(trifluoromethanesulfonyl)imide [C1(C2H2CH)Im][NTf2], 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C1C3CNIm][NTf2], 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide [C1C3CNIm][DCA], 1-butyl-3-methylimidazolium dicyanamide, [C1C4Im][DCA] and 1-butyl-3-methylimidazolium methylphosphite [C1C4Im][C1HPO3]. The densities and viscosities of the ionic liquids studied are strongly dependent on the anion and, at 313 K, follow the order: [C1(C2H2CH)Im][NTf2] > [C1C3CNIm][NTf2] > [C1C3CNIm][DCA] > [C1C4Im][C1HO3] > [C1C4Im][DCA] and [C1C3CNIm][NTf2] > [C1C3CNIm][DCA] > [C1C4Im][C1HPO3] > [C1(C2H2CH)Im][NTf2] > [C1C4Im][DCA], respectively. The differences in the molecular structures of the ionic liquids allowed the identification of the influence of increasing the alkyl side chain of the cation, of the presence of unsaturated Ctriple bond; length of mdashC and Ctriple bond; length of mdashN bonds on the alkyl side chain of the cation and finally of a phosphite based anion on the selective absorption of ethane and ethylene. The solubility of ethylene is higher than that of ethane in the ionic liquids studied and varies from 18.41 × 10−3 in [C1C8Im][NTf2] at 303.17 K to 1.603 × 10−3 in [C1C4Im][DCA] at 343.61 K. The introduction of the different functional groups leads to a decrease on the gas absorption compared with that of [C1C8Im][NTf2] but to an increase of the ideal separation selectivity of ethane and ethylene.