Fluidized beds are interesting and useful processing systems that are employed in many industries such as processing biomass into biofuels or the coating of pharmaceuticals. Knowledge of fluidized bed hydrodynamics is necessary for the design and scale-up of such devices. This paper describes the local time-average differences of gas holdup in a 10.2 cm and 15.2 cm diameter cold flow fluidized bed that were recorded using 3D X-ray computed tomography. Three different Geldart type B bed materials are studied at various superficial gas velocities and side-air injection flow rates, where the side-air injection simulated the immediate volatilization of a fuel like coal or biomass particles. Variations in side-air injection flow rate have little influence on global bed hydrodynamics, but significantly affects local gas holdup. Axial annular flow dominates over all flow conditions for each material and bed diameter. Wall effects increasingly influence hydrodynamics as bed diameter decreases for all materials. (C) 2011 Elsevier Ltd. All rights reserved.