Abstract: The Clegg - Pitzer equations with some simplifications are used to correlate the vapor - liquid equilibrium data for the CO2 - MDEA - H2O and CO2 - MEA - H2O systems. The interaction parameters determined from the data for binary .ternary systems can be used to predict the quaternary systems without any additional parameters. The model is applied to the CO2 - MDEA - MEA - H2O system containing three neutral solvents (MDEA,MEA and H2O) and four ionic species (MDEAH+, MEAH+, HCO3- and RNHCOO-) with four chemical equilibria simultaneously involved.