Removal of acid gases from natural and flue gases is vital in gas treatment processes. Using a high pressure cell, the (vapor + liquid) equilibrium data for solubility of CO2 in the aqueous mixtures of diisopropanolamine (DIPA) + 2-amino-2-methyl-1-propanol (AMP) + piperazine (PZ) are obtained at 313.15, 328.15 and 343.15 K and pressure range of 1-40 bar. Also the experimental measurements for solubility of CO2 in the aqueous (DIPA + AMP) -(DIPA + PZ) systems and the binary aqueous DIPA system are carried out at the same conditions as above. The compositions of the quaternary aqueous mixtures are: (36 wt.% DIPA + 7 wt.% AMP + 2 wt.% PZ), (30 wt.% DIPA + 10 wt.% AMP + 5 wt% PZ), (24 wt.% DIPA + 13 wt.% AMP + 8 wt.% PZ), for the ternary mixtures are: (24 wt.% DIPA + 21 wt% AMP), (24 wt.% DIPA + 21 wt.% PZ) and for the binary mixture is 45 wt.% DIPA, keeping the total aqueous amine concentration at 45 wt.%. The results are presented as the partial pressure of CO2 against acid gas loading (mol CO2/mol total amine). It's observed that using the AMP-PZ blend as an activator for the DIPA solution increases the CO2 loading so that the absorption of CO2 enhances in the aqueous DIPA solution. Moreover, the Electrolyte-NRTL model is used to model the experimental data of this work and the other aqueous alkalonamine + acid gas mixtures from the literature data. The percent of the average absolute deviation, AAD%, between the calculated partial pressure of CO2 and experiment is 20.3 for the quaternary aqueous-alkanolamines mixtures. (C) 2013 Elsevier Ltd. All rights reserved.