DURING a preliminary investigation of mass transfer from bubbles in a fluidized bed composed of cracker catalyst (mean particle size 60µ), it was noticeable that the presence of bubbles caused a reduction of the height of the bed which was obtained when either a bubble flowstream or individual bubbles were injected into the bed. It appears that gas flows preferentially from the continuous phase into the bubble, and that the hold-up of gas in the continuous phase is reduced because the rising velocity of the bubbles is about 100 times that of the gas in the continuous phase.