Three cheap DESs comprising of N-methyldiethanolamine (MDEA) and imidazole (Im), 1,2,4-triazole, and tetrazole were investigated for capturing SO2 at low concentrations. Surprisingly, with the addition of Im, the SO2 absorption capacity and desorption efficiency were improved. Spectroscopic analysis and quantum chemical calculations confirmed that MDEA–Im effectively and reversibly captured SO2 through the hydrogen bond network and synergistic action between MDEA and Im.