This work presents new experimental results for the CO2 solubility in aqueous blends of diethanolamine (DEA) + N-methyl diethanolamine (NIDEA) in the temperature range of 303-323 K and CO2 partial pressure of 1-100kPa. The concentrations of the aqueous amine blends were 1.5 mass% DEA + 28.5 mass% MDEA, 3 mass% DEA + 27 mass% MDEA, and 4.5 mass% DEA + 25.5 mass% NIDEA. A rigorous thermodynamic model is developed to correlate and predict the vapour-hquid equilibrium (VLE) of CO2 in aqueous blends of DEA + MDEA. The modified Clegg-Pitzer equations have been used to develop the model for the quaternary system (CO2 + MDEA + DEA + H2O) using the interaction parameters derived from the VLE data of the corresponding ternary systems CO2 + MDEA + H2O and CO2 + DEA + H2O. Simulated annealing, a non-traditional algorithm, has been used for parameter estimation and for the determination of equilibrium composition of various species present in the liquid phase. The model predictions have been in good agreement with the experimental data of CO2 solubility in DEA + MDEA blends of this work as well as those reported in the literature. (c) 2006 Elsevier B.V. All rights reserved.