The absorption performance of CO2 in the ionic liquid (IL) aqueous solutions: tetramethylammonium glycinate [N-1111][Gly] + N-methyldiethanolamine (MDEA) was investigated in detail at 25 degrees C by using constant volume method. To explore the effect of the solution's compositions on the absorption of CO2, fifteen CO2 absorbents were prepared by varying the concentration of [N-1111][Gly] in the range of 2.5-15 wt% or MDEA's concentration ranging from 30 wt% to 50 wt%. The results revealed that increase in IL's concentration in MDEA aqueous solutions had conflicting influence on the CO2 absorption rate. By adding ILs to MDEA of a fixed concentration, the minimum and maximum of apparent absorption rate K-0 approached at 5 wt% IL and 10 wt% IL, respectively. It was also found that MDEA dramatically increased the absorption load of CO2 in ILs aqueous solutions, while the presence of ILs in the solvent system had little effect on the absorption load of MDEA solutions. This exactly testified to the previously proposed theory [1] of the absorption of CO2 in amino acid based ILs + MDEA aqueous solutions. Moreover, the physical properties necessary for the transfer characterization, such as density, viscosity and surface tension of the solution, were measured for the solvents to indicate the regularity. (C) 2011 Elsevier B.V. All rights reserved.