Gaseous solubilities of carbon dioxide (CO2) in 18 room-temperature ionic liquids (RTILs) have been measured at an isothermal condition (about 298 K) using a gravimetric microbalance. The observed pressure-temperature-composition (PTx) data have been analyzed by use of an equation-of-state (EOS) model, which has been successfully applied for our previous works. Henry’s law constants have been obtained from the observed (PTx) data directly and/or from the EOS correlation. Ten RTILs among the present ionic liquids results in the physical absorption, and eight RTILs show the chemical absorption. The classification of whether the absorption is the physical or chemical type is based on the excess Gibbs and enthalpy functions as well as the magnitude of the Henry’s constant. In the chemical absorption cases, the ideal association model has been applied in order to interpret those excess thermodynamic functions. Then, two types of the chemical associations (AB and AB2, where A is CO2 and B is RTIL) have been observed with the heat of complex formations of about −11 (for AB) and from −27 to −37 (for AB2) kJ·mol−1, respectively.