The hydrodynamics have been studied in a cold, freely bubbling, pressurized fluidized bed. The bed has a cross-section of 0.2 m;0.3 m and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s. The bed material was silica sand with a mean particle diameter of d N "0.45 mm. Comparisons were made with previous results obtained with particles of d N "0.7 mm. The hydrodynamic results are similar for the two different particle sizes when plotted vs the excess gas velocity. The results also show that the bed expansion, bubble rise velocity, bubble volume fraction and visible bubble flow rate fall on single curves if plotted vs a dimensionless potentially available drag force, while the bubble frequency, the mean pierced length and the through-flow velocity of gas through the bubble do not. The dimensionless drag force is a suitable scaling parameter as long as the particles do not respond to the gas-phase velocity fluctuations and as long as the dense phase does not expand. At high pressures, an increased gas-particle interaction, in combination with turbulent fluctuations in the gas phase, can be used to explain the increased bubble instability, with a corresponding increased bubble splitting and dense phase expansion. The gas-particle interaction also increases with decreasing particle size, which may help explain the maximum stable bubble size for group A particles observed by many workers.