Room-temperature ionic liquids (ILs) are widely investigated to absorb SO2 from mixed gases or simulated flue gases and can capture a large amount of SO2, which can be recovered easily by heating and vacuum treatment. However, due to the existence of O-2 in flue gases, the oxidation of SO2 may occur. This oxidation might influence the SO2 recovery from ILs and make ILs unable to reuse. This may limit further applications in large-scale desulfurization from mixed gases or flue gases by ILs. In this work, a task-specific IL, monoethanolaminium lactate ([MEA]L), was used to study the absorption of SO2 and oxidation of SO2 by O-2 in simulated flue gases with and without ash and activated carbon in [MEA]L. It is found that the presence of O-2 in the simulated flue gas does not influence the absorption of SO2 by [MEA]L, but it causes, to a very small extent, the oxidation of SO2. The increase of temperature, time, and the concentration of O-2 can increase the oxidation of SO2. Adding in IL ash and activated carbon, which could be captured by IL from flue gases, can also increase the oxidation of SO2. For these problems, we have tried to find some ways to reduce the oxidation of SO2 absorbed by IL.