The gas holdup, epsilon, and volumetric mass transfer coefficient, k(L)a, were measured for the air-water system in bubble columns of three different diameters, 0.1, 0.15 and 0.38 m. The superficial gas velocity U was varied in the range 0-0.35 m s(-1), spanning both the homogeneous and heterogeneous flow regimes. The gas holdup epsilon shows a small decrease with increased column diameter; this effect is due to increased liquid recirculations with increasing scale. The volumetric mass-transfer coefficient, k(L)a, closely follows the trend in gas holdup. For U > 0.08 m s(-1), the value of k(L)a/epsilon was found to be practically independent of column diameter and superficial gas velocity; the value of this parameter is found to be about 0.48 s(-1). Our studies provide a simple method for estimation of k(L)a in industrial size bubble columns operated at high superficial gas velocities. (C) 2003 Elsevier B.V. All rights reserved.