Local time-averaged gas holdup in a 10.2 cm diameter fluidized bed is determined using X-ray computed tomography (CT) over a range of superficial gas velocities (U-g), side air injection flow rates (Q(side)), and fluidized bed material. Without side air injection, only small variations in the local time-averaged gas holdup are observed for beds composed of glass beads, ground walnut shell, or ground corncob. With the introduction of side air injection, which simulates the immediate volatilization of biomass in a fluidized bed gasifier, a distinctive plume is observed along the reactor wall above the side injection port. The plume gradually expands toward the center of the bed as height increases; the expansion is found to increase with increasing Q(side). As U-g increases, fluidization becomes more uniform and the effect of the side air injection on the fluidization hydrodynamics is less pronounced. Additionally, increasing U-g increases overall gas holdup and bed expansion. Of the three bed materials examined, ground corncob fluidization is the least affected by side air injection and shows the highest overall gas holdup while glass bead fluidization is much more affected by side air injection and has the lowest overall gas holdup. This study demonstrates the usefulness of X-ray CT in noninvasively visualizing detailed internal features of fluidized beds. These results will be used in future studies to validate computational fluid dynamics (CFD) models of fluidized beds. (C) 2009 Elsevier B.V. All rights reserved.