Experimental data on the average mass transfer liquid film coefficient (k(L)) in an aerated stirred tank are presented. Liquid media used were tap water, electrolyte solutions and water with controlled addition of tensioactive material. Values of k(L) range from those expected for bubbles with a mobile surface to those expected for rigid bubbles. These data are quantitatively interpreted in terms of bubble contamination kinetics, using a stagnant cap model, according to which bubbles suddenly change from a mobile interface to a rigid condition when surface tension gradients, caused by surfactant accumulation, balance out shear stress. (C) 2003 Elsevier B.V. All rights reserved.