Gaseous solubilities of sulfur dioxide (SO2) in room-temperature ionic liquids (RTILs), 1-n-butyl-3-methylimidazolium acetate and 1-n-butyl-3-methylimidazolium methyl Sulfate, have been measured at four isothermal conditions (about 283, 298, 323, and 348 K) using a gravimetric microbalance. The observed pressure-temperature-compositional (PTx) data have been analyzed by use of an equation-of-state (EOS) model, which has been Successfully applied for our previous works. Excess thermodynamic functions and Henry's law constants have been obtained from the observed (PTx) data and our previous measurements of SO2 + 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide using the EOS correlation. All three RTILs show the chemical absorption. The classification of whether the absorption is the physical Or chemical type is based oil the excess Gibbs and enthalpy functions as well as the magnitude of the Henry's constant. An ideal association model has been applied in order to interpret those excess thermodynamic functions. Then, two types of the chemical associations (AB and AB(2), where A is RTIL and B is SO2) have been observed with the heat of complex formations of about -6 to -19 (for A B) and from -6 to -29 (for AB(2)) kJ.mol(-1), respectively.