AbstractThe published literature on gas mass transfer in three‐phase fluidized beds indicates that most of the research done in the last two decades has been carried out at low superficial velocities and/or volumetric flow rates. These experimental limitations are reflected in the current understanding of these systems. In this investigation, performed at large gas and liquid velocities and flow rates, the behaviour of three‐phase fluidized beds of 0.1, 0.3 and 0.5 cm diameter glass beads has been studied.Computer‐made contour diagrams, obtained from the mapping of the oxygen concentration in the column have proven to be an effective probing technique. These diagrams permit the description of flow patterns and bed hydrodynamic properties otherwise not observable with tracers.The use of concentration and pressure profiles complements the observations made with the contour diagrams. At low gas and liquid velocities the concentration profiles areS‐shaped. AsVgandVlincrease the three absorption zones, observed in thoseS‐shaped profiles, become two zone similar to those observed in bubble columns. The interface between the dense bed and dilute phase is characterized by an unusually high mass transfer