An electrochemical method was used to follow CO2 absorption both in water and in alkanolamine solutions in a bubble column (-1 m tall). This method allows the determination of local mass transfer coefficients along the column. No special care was taken in avoiding trace contaminants. It was found that bubbles contaminate mostly at the gas distributor. Gas-liquid mass transfer coefficient decreases as bubbles rise along the column, taking values closer to those expected for clean bubbles with a mobile surface at the bottom of the column, and values closer to those expected for rigid bubbles, at the top of the column. If this is quantitatively interpreted within the framework of the stagnant cap model, it may be concluded that this decrease is mainly due to bubble shrinkage, which leads to a greater fraction of bubble area being covered by the stagnant cap. Compared with this effect. the effect of further acquisition of contaminant molecules by the bubbles is negligible. The above conclusions can be drawn both for absorption with chemical reaction in amine solutions and for pure absorption of CO, in tap water, although in this case the shrinkage effect is less pronounced. (C) 2007 Elsevier B.V. All rights reserved.