The solubility of pure CO2, CH4, and N2 in the mixture of choline-2-pyrrolidine carboxylic acid (Cho­Pro) and polyethylene glycol (PEG200) (mass ratio = 1:2) was measured experimentally at temperatures from 308.15 to 338.15 K and pressures up to 28 bar, in which Cho­Pro is an ionic liquid and PEG200 is a cosolvent with the purpose to decrease the viscosity. It was found that Cho­Pro/PEG200 showed a good selectivity for CO2/CH4 and CO2/N2 separation. The measured experimental data points from this work and others were further used to estimate the thermodynamic properties including the Henry’s law constants for the gases in Cho­Pro/PEG200, the equilibrium constant for the reaction between CO2 and Cho­Pro, the CO2 absorption enthalpy in Cho­Pro/PEG200, and so forth. The consistent results of the CO2 absorption enthalpy at infinite dilution prove the reliability of the thermodynamic properties obtained in this work. The thermodynamic properties of Cho­Pro/PEG200 were further compared with other three typical absorbents, and the absorption enthalpy is nearly half of that for 30 wt % MEA aqueous solution. At the same time, the theoretical amount of absorbents needed for Cho­Pro/PEG200 is much lower than that of H2O scrubbing. This shows that Cho­Pro/PEG200 is a promising absorbent.