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The electrolyte SAFT-HR (eSAFT-HR) equation of state (EoS) has been successfully applied to model the solubility of CO 2 in aqueous N-methyldiethanolamine (MDEA) solutions in a wide range of temperature (298–413 K), pressure (2.84–7565 kPa), MDEA concentration (18.8–50.1 wt%), and CO 2 loading (0.00025–1.31608). A consistent database including 413 experimental data points is used in this study. The eSAFT-HR EoS is composed of the SAFT-HR EoS, the explicit simple version of the mean spherical approximation (MSA) theory and the Born equation. Vapor–liquid equilibrium calculations are performed to adjust the binary interaction parameters (BIP) for H 2 O–MDEA and H 2 O–CO 2 binary solutions. Having the BIPs between the molecular species, the solubility of CO 2 in the ternary aqueous solution of MDEA is calculated by simultaneous vapor–liquid and chemical equilibrium calculation algorithm. Also, to improve the results of the solubility of CO 2 in the ternary CO 2 –MDEA–H 2 O systems, two additional BIPs ( k MDEA H + − MDEA , k MDEA H + − C O 2 ) are optimized using eSAFT-HR EoS. To decide which BIP has the most influence on the results a sensitivity analysis is made. Since the ion MDEAH + does not exist in the binary solutions, these parameters are adjusted to the solubility data of the ternary solution. The BIPs are considered to be temperature dependent. For the ternary solution the overall average absolute percent relative deviation (AAD%) and mean BIAS percent deviation (BIAS%) are 11.2 and 4, respectively. To verify the ability of the model, the eSAFT-HR EoS is used to model the solubility data of Huttenhuis et al. (2007) for the MDEA–CO 2 –H 2 O system at the temperature of 298 K. The obtained AAD% and BIAS% are 25.6% and 5.5%, respectively. The results obtained by the eSAFT-HR EoS are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]