The products and pathway for the oxidative degradation of CO2-loaded and concentrated aqueous solution of monoethanolamine (MEA)/methyl diethanolamine (MDEA) mixture (i.e., MEA-MDEA-H2O-CO2 system) were evaluated and compared with those for the MEA-H2O-CO2 system in a stirred cell reactor at temperatures in the range of 55-120 degrees C, overall amine concentration in the range of 5-9 mol/L, MDEA/MEA ratio of 0-0.4, CO2 loading in the range of 0-0.53 mol/mol of total amine, and O-2 pressure of 250 kPa in order to determine the role of MDEA in preventing MEA degradation. The results showed that fewer degradation products were obtained for the MEA-H2O-O-2 system for both the CO2-loaded and CO2-free cases as compared with the MEA-MDEA-H2O-O-2 system. However, the addition of MDEA drastically reduced the extent of MEA degradation as well as the amount of nonenvironmentally benign degradation products. Our overall results indicate that, under our experimental conditions, MDEA is more prone to oxidative degradation and, when used in a mixture with MEA, is preferentially degraded to protect MEA. Our results further show that even in an initially O-2-free environment, O2 is produced as a byproduct Of CO2-induced degradation, thereby eventually generating an oxidative degradation environment for the two systems.