Room-temperature ionic liquids (ILs) have been demonstrated to absorb SO2 efficiently. However, after absorbing a large amount of SO2, the viscosity, the conductivity, and the density of the ILs have not been studied systematically, and the mechanism of the interaction between SO2 and ILs is still being disputed. In this work, two kinds of ILs (task-specific ILs and normal ILs) have been studied to absorb pure SO, at atmospheric pressure. It is found that the viscosity, the conductivity, and the density show different behaviors between task-specific ILs and normal ILs. For the task-specific ILs to absorb SO2, before a 0.5 mol ratio of SO2 to IL, the viscosity and density increase, and the conductivity decreases with an increase of the mole ratio of SO2 to IL. After that, the conductivity and density increase, and the viscosity decreases with further increasing the mole ratio of SO2 to IL. However, for the normal ILs, the conductivity and density increase and the viscosity decreases with an increase of the mole ratio of SO2 to IL. A new mechanism of ILs absorbing SO2 has been proposed. Task-specific ILs can chemically absorb SO2 when the mole ratio of SO2 to IL is not more than 0.5, and they can physically absorb SO2 when the mole ratio is more than 0.5. The normal ILs can only physically absorb SO2.