Task-specific ionic liquids (TSILs) have been experimentally demonstrated to absorb more sulfur dioxide (SO2) than normal ILs from gas mixtures with low SO2 concentrations; however, the differences of SO2 solubilities in the two kinds of ILs at given temperatures and pressures have not been studied systematically. Moreover, the mechanism of the interaction between SO2 and ILs still remains unclear. In this work, the solubilities of SO2 in TSILs (1,1,3,3-tetramethyl-guanidinium lactate and monoethanolaminium lactate) and normal ILs (1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophovhate) were determined. The solubilities of SO2 are correlated by a modified Redlich-Kwong equation of state (RK EoS). The chemical absorption and physical absorption are differentiated, and the absorption mechanism has been proposed with the aid of the modified RK EoS. SO2 absorption capacity in TSILs is contributed from both chemical interaction and physical interaction. Two TSIL molecules chemically absorb one SO2 molecule, and the chemical absorption amount follows the chemical equilibrium. Normal ILs only physically absorb SO2 following Henry's law. The chemical equilibrium constant, reaction enthalpy, Gibbs energy of reaction, reaction entropy, and Henry's law constant of SO2 absorbed in ILs have been calculated. The present model can predict SO2 absorption capacity for capture and SO2 equilibrium concentration in IL for recovery.