Experimental studies on bubble sizes in a high-intensity gas/liquid contactor are described. The contactor utilizes cross flow of liquid at high velocities to generate small bubbles. The relatively limited previous literature on bubble formation in cross flow is first reviewed. Measurements using a video technique at the exit of a short version of the contactor and on a section of the porous distributor are then described. The latter allows study of individual pores. Data on the influence of water velocity and gas rate on mean sizes and size distributions are presented. Electrolytes added to the water allow the significance of bubble coalescence to be quantified. The critical concentrations for complete suppression of coalescence are compared with theoretical values from two models. Aspects of predictive modelling are also considered briefly.