The study of mass transfer (oxygen or ozone) from gas bubbles to a liquid phase is based on experiments and modelling. Two pilot plants have been used : a deep U-tube (DUT) and a classical bubble column. The experiments were carried out in the bubbly flow pattern. The void fraction, the pressure gradient, the bubble diameter and the volumetric mass transfer coefficient were measured. A two-fluid model was developed and validated in comparison to the experimental data. A key point of the hydrodynamic model lies in the interfacial momentum transfer. The prospect being the modelling of mass transfer, we propose to relate the mass transfer coefficient to a velocity scale which accounts for interfacial momentum transfer.