The Danckwerts-plot technique [Danckwerts et al., 1963. Chemical Engineering Science 18, 63-72] is used in chemical engineering to simultaneously obtain the mass transfer parameters, k(L) and a, from mass transfer experiments. This method requires variation of the reaction kinetics by adding different amounts of catalyst. Although the method is known for several decades, it was never verified that the variation of the amount of catalyst does not affect the hydrodynamics of the system under investigation. To study this, absorption Of CO2 in a carbonate/bicarbonate buffer solution was performed simultaneously with desorption of oxygen from this solution, after verification that absorption and desorption are processes, taken place at identical rates, but in a different direction. It was shown that the addition of catalyst did not affect the desorption rate of oxygen. The obtained k(L)a for oxygen was, however, 64% higher compared to the kLa of carbon dioxide. This was probably due to a lower effective interfacial area, caused by the complete depletion of small bubbles containing CO2. Mass transfer experiments with oxygen, with a low gas phase conversion, are therefore to be preferred, as the measured mass transfer parameters are less affected by the gas phase RTD and the shape of the bubble size distribution. (c) 2005 Elsevier Ltd. All rights reserved.