The degradation of aqueous diethanolamine (DEA) solutions by carbonyl sulfide was examined by using a 600-mL well-stirred reactor operating under the following conditions : DEA concentration 20-40 wt %, temperature 120-180-degrees-C, COS partial pressure 0.3-1.17 MPa. The reaction products were identified by GC/MS, and reaction mechanisms are developed which conform with experimental observations. The reaction rate constants are determined, and a mathematical model for estimating DEA degradation by COS is presented.