Using a fast X-ray tomography setup the bubble size and velocity has been measured in a 25 cm diameter fluidized bed of Geldart B powder and a 24 cm bed of Geldart A powder. The average bubble size has been determined for a measurement period of 60 s. The resolution of this setup is about 4.5 mm per pixel at a rate of 250 reconstructions per second. It is possible to detect bubbles as small as 2.2 cm. The Geldart B powder consists of polystyrene particles with an average diameter of 607 gm and a bulk density of 625 kg/m(3). This bed was studied at pressures ranging from 1 to 5 bar. The superficial gas velocities varied from 12 to 32 cm/s for the atmospheric pressure measurements; for the highest pressure from 10 to 15 cm/s. The bubble size is significantly reduced at higher pressures for similar gas flows. The Geldart A powder consist of a base of aluminum oxide particles with an average diameter of 76 mu m and a bulk density of 680 kg/m(3). A varying amount of fines was added to these base particles for the different mixtures. The fines consist of aluminum oxide particles with an average diameter of 38 mu m and a bulk density of 620 kg/m(3). The fines contents varied from 0%(w) to 50%(w). An increase in fines content results in a clear reduction of the average bubble size. If the fines content is increased from 0%(w) to 50%(w) the average spherical equivalent bubble diameter is reduced by 20%. (C) 2012 Elsevier B.V. All rights reserved.