In order to be able to understand and compensate for the mass transfer effects on the reaction kinetics that typically occur in these studies, the influences of diffusion and gas phase resistance on the measurement of kinetics have been investigated. Carbon dioxide (CO2) absorption into monoethanolamine (MEA) and N-methyldiethanolamine (MDEA) have been taken as the fast and slow reaction cases in this work. The inflection point between diffusion control and reaction control regions was found in the fast reaction case but not in the slow one. Further, there is a low critical value of P-inert, below which its effect can be ignored, in the fast reaction system. Conversely, the critical value of P-inert in the slow reaction system is higher. Additionally, credible intrinsic kinetics of the both systems have been obtained under optimum conditions, expanding the temperature range to lower than what is available in the literature.