The reaction between CO2 and diethanolamine (DEA) in aqueous ethyleneglycol (ETG) at 298 K has been studied over the complete composition range. The application of this reaction as a viscous gas—liquid system for the determination of interfacial areas in gas—liquid contactors by the chemical method is discussed. The reaction kinetics have been determined by mass transfer experiments of CO2 into solutions of DEA in aqueous ETG. To this end laboratory-scale stirred cell reactors with a flat surface have been used. In accordance with the same reaction in water at 298 K the reaction between CO2 and DEA in aqueous ETG at 298 K can be described by the zwitterion mechanism of Caplow. Special attention has been paid to the reversibility of the reaction between CO2 and DEA. Calculation show that the influence of the reversibility on the mass transfer rate can be neglected for partial pressures of CO2 below 3 kPa. It is demonstrated that the reaction between CO2 and DEA in aqueous ETG can be used for the determination of interfacial areas in gas—liquid contactors at higher viscosities.