In this work, the, effects of surface-active contaminants on mass transfer coefficients k(L)a and k(L) were studied in two different bubble contactors. The oxygen transfer coefficient, k(L), was obtained from the volumetric oxygen transfer coefficient, k(L)a, since the specific interfacial area, a, could be determined from the fractional gas holdup, epsilon, and the average bubble diameter, d(32). Water at different heights and antifoam solutions of 0.5-100 ppm were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and 0.85 x 10(-3) m(3) capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k(L)a and k(L) values over a span of almost 400%. A theoretical interpretation is proposed based on modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant cap model. Model results match experimental k(L) data within +/-30%. (C) 2003 Elsevier Science Ltd. All rights reserved.