The objective of this study was to determine the correlation equation between the CO2 absorption capacity and measured electrical conductivity in a CO2 absorption system using diethanolamine (DEA) as a solvent based on various theoretical and experimental results. In addition, the capacity and various results of the system were quantified and compared to those of the monoethanolamine (MEA) system reported in previous papers. DEA solvent at 4 M was confirmed to be the most suitable one for application in a commercial process because its mole ratio of CO2 to amine was approximately 0.5. The ionic conductivities of carbamic acid (R2NH2+) and carbamate (R2NCOO-) were estimated to be 38.46 and 20.01 Scm(2)mol(-1)z(-1), respectively. They were smaller than those of their corresponding ions of the MEA system. The ionic activity coefficient of the DEA system was also smaller than that of the MEA system. However, the sensitivity of the CO2 absorption capacity to electrical conductivity measured in a constant-concentration solution of the DEA system was higher than that in the MEA system. These results were basically caused by differences in molecular size, the mole ratio of water to all ions present (H2O/ions), and the density of the electron dispersion of molecules between the MEA system and the DEA system.