The sizes of bubbles produced in a bed of 2.3 mm diameter glass particles fluidised by a hydrocarbon oil have been measured photographically. The gas was introduced in two ways, either as a continuous stream or injected as a pulse to form a large bubble. As the voidage of the bed was increased from 0.42 to 0.85, the character of the gas flow changed from the coalesced bubble regime to the dispersed bubble regime, which was the purpose in selecting the particular hydrocarbon liquid chosen. From the theoretical treatment described, the velocity with which a particle impacts the upper surface of an spherical cap bubble was computed. It is suggested that the transition between the two gas flow regimes might be characterised by a Weber number based on this impact velocity.