In this work, the solubility of CO2 in aqueous blended systems of N-methyldiethanolamine (MDEA) and guanidinium trifluoromethanesulfonate ([gua](+)[OTf](-)) have been carried out. The solubilities were measured at temperature 303.2 K, 323.2 K and 333.2 K, over CO2 partial pressure ranging from 500 to 3000 kPa. All data were reported as loading capacity (mol CO2/total mol) as a function of partial pressure of CO2 at the corresponding temperature. It has been found that the aqueous [gua](+)[OTf](-) ionic liquid gave significantly higher solubility, up to 1.63 mol CO2/total mol, as compared to other pure ionic liquids such as [bmim](+)[BF4](-), [emim](+)[OTf](-) and [emim](+)[C2N3](-) which recorded at 323.2 K and 3000 kPa. However, the addition of [gua](+)[OTf](-) ionic liquid to aqueous MDEA gave a slight decreased on solubility. Correlations of solubility as a function of partial pressure and temperature were obtained with deviation of +/-0.957%. (C) 2010 Elsevier B.V. All rights reserved.