Alkyl amines, which have multiple amino groups, are used as activators to improve the CO2 absorption performances of aqueous methyldiethanolamine (MDEA) solutions. The aqueous MDEA blends consisted of 20% (w/w) of MDEA and 10% (w/w) of activators, which are 3-methylamino propylamine (MAPA), diethylenetriamine (DETA), triethylene tetramine (TETA), and tetraethylenepentamine (TEPA). Aqueous solutions of monoethanolamine (MEA; 30% (w/w)) and MDEA (30% (w/w)) are used as reference absorbents for comparison. The CO2 absorption performances of aqueous MDEA blends were investigated by measurements of absorption capacities, absorption rates, and heats of absorption. The MDEA blends have higher CO2 absorption capacities than MEA and MDEA. MDEA/TEPA shows the highest CO2 loading amount of 0.753 mol-CO2 center dot mol-absorbent(-1) at 313 K. In addition, the MDEA blends show high cyclic capacities (0.241-0.330 mol-CO2 center dot mol-absorbent(-1)), the values of which are about 3 times higher than that of MEA. All MDEA blends show higher absorption fluxes than MDEA. The MDEA/MAPA showed the highest overall mass transfer coefficient of 3.351 x 10(3) mol.m(-2).s(-1)-kPa(-1), 8 times higher than that of MDEA (0.451 x 10(3) mol.m(-2)-s(-1).kPa(-1)) and even higher than that of MEA (3.014 x 10(3) mol.m(-2)-s(-1)-kPa(-1)). The heats of absorption of the MDEA blends (57.21-59.53 kJ.mol.CO2-1) are about 30% higher than that of MDEA and about 30% lower than that of MEA.