We consider the liquid-side mass transfer coefficient k(L) in a dense bubble swarm for a wide range of gas volume fraction (0.45% <= alpha(G) <= 16.5%). The study is performed for an air-water system in a square column. Bubble size, shape and velocity have been measured for different gas flow rates by means of a high speed camera. Gas volume fraction and bubble velocity have also been measured by a dual-tip optical probe. Both of these measurements show that the bubble vertical velocity decreases when increasing alpha(G) in agreement with previous investigations. The mass transfer is measured from the time evolution of the dissolved oxygen concentration, which is obtained by the gassing-out method. The mass transfer coefficient is found to be very close to that of a single bubble provided the bubble Reynolds number is based on the average equivalent diameter < d(eq)> and the vertical slip velocity < V-z >. (C) 2011 Elsevier Ltd. All rights reserved.