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% ≤ α 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 α 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 .