The new group binary interaction parameters of UNIFAC model (a(nm) and a(mn)) between CO2 and 22 ionic liquid (IL) groups were obtained by means of correlating the solubility data of CO2 in pure ILs at different temperatures (>273.2 K). We measured the CO2 solubility at low temperatures down to 243.2 K in pure ILs, i.e., [OMIM](+)[BF4](-) and [OMIM](+)[Tf2N](-), and their equimolar amount of mixture, in order to fill the blank of solubility data at low temperatures and also to justify the applicability of UNIFAC model over a wider temperature range. It was verified that UNIFAC model can be used for predicting the CO2 solubility in pure ILs and in the binary mixture of ILs both at high (>273.2 K) and low temperatures (<273.2 K) effectively, as well as identifying the new structure-property relation. This is the first work to extend the UNIFAC model to IL-CO2 systems. (c) 2013 American Institute of Chemical Engineers AIChE J 60: 716-729, 2014