Ionic liquids are promising solvents for extracting CO2 from H2, N2 and O2 using the absorption separation method. Solubilities of these gases in 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM][eFAP]) were determined using an isochoric saturation method. Experimental result indicates that solubilities of H2, N2 and O2 in [HMIM][eFAP] are much lower than that of CO2. The performance of [HMIM][eFAP] in separating CO2 from N2, H2 and O2 can be improved by decreasing the temperature. Based on the solubility data, the ideal selectivities of CO2/N2, CO2/H2, and CO2/O2 in different ionic liquids were calculated and the effect of the ionic liquid structure on them were investigated.