New 1,1,3,3-tetramethylguanidinium-based ionic liquids have been synthesized and their ability to reversibly absorb gaseous sulfur dioxide and ammonia investigated. It was found that up to 2 moles of gas at 1 bar could be absorbed per mole of ionic liquid at room temperature and almost completely desorbed by heating or lowering of the pressure. No change in absorbing capacity of the ionic liquids was observed after several cycles. The absorption of SO2 in the ionic liquids is an exothermic process and standard enthalpy of solution was found to be in the range −21 to −37 kJ mol−1, indicating a moderate association between gas and liquid, which also could be confirmed by Raman and UV–vis spectroscopy on the gas-saturated (1 bar) ionic liquids. Knowledge of gas solubilities and the reversible gas absorption capacity of ionic liquids are believed to have importance for their possible application as reaction media for, e.g. catalytic processes involving gaseous reactants, and as absorbents in gas separation processes.