The solubility of CO2 in the mixture of aqueous monoethanolamine (MEA) with benzoic acid was measured and correlated using a reaction-phase equilibrium model. The solubility of CO2 in 15 wt % aqueous MEA solution was measured at a temperature of 313.2 K and pressures up to 200 kPa. Also, the solubility of CO2 in the mixtures with various weight ratios of benzoic acid, MEA, and water were measured at a temperature of 313.2 K and pressures ranging from (1 to 250) kPa. The weight ratios (acid/MEA/water) were 1:15:85, 2:15:85, 10:15:85, 15:15:85, 1:14:85, 2:13:85, and 5:10:85. Reaction equilibrium constants of the Kent-Eisenberg model were used for the CO2-MEA-water mixture, and new equilibrium constants involving benzoic acid were estimated using experimental data. The concentration of species in the liquid phase was estimated as a function of CO2 loading, and the effect of adding benzoic acid can be predicted within reasonable accuracy using the proposed model.