In this study the electrolyte equation of state as proposed by Solbraa [E. Solbraa, Equilibrium and non-equilibrium thermodynamics of natural gas processing, Ph.D. thesis Norwegian University of Science and Technology, 2002] was systematically studied and improved to describe the solubility of carbon dioxide in aqueous solutions of N-methyldiethanolamine quantitatively. In this electrolyte equation of state approach both the vapour phase and the liquid phase are described with an equation of state. The molecular part of the equation is based on Schwarzentruber's [J. Scharzentruber, H. Renon, S. Watanasiri, Fluid Phase Equilib. 52 (1989) 127-134] modification of the Redlich-Kwong equation of state with the Huron-Vidal mixing rule [M.J. Huron, J. Vidal, Fluid Phase Equilib. 3 (1979) 255-271]. Three ionic terms are added to this equation-a short-range ionic term, a long-range ionic term (MSA) and a Born term. The thermodynamic model has the advantage that it reduces to a standard cubic equation of state if no ions are present in the solution, and that publicly available interaction parameters used in the Huron-Vidal mixing rule can be utilized. In this work binary molecular- and ionic interaction parameters were studied and optimized. With the updated model it was possible to describe both low- and high pressure vapour-liquid-equilibrium for the MDEA-H2O-CO2-CH4 system satisfactorily. (c) 2007 Elsevier B.V. All rights reserved.