X-ray imaging was employed to visualize the internal flow structures of the travelling fluidized bed (TFB), operating in the bubbling and turbulent flow regimes. The degree of interference of a 4-mm diameter intrusive probe inserted in the gas-fluidized bed was investigated by comparing the time-average voidage in a region of the bed operated with and without the probe being present. Transition from bubbling to the turbulent fluidization regime was visually inspected from images obtained by the X-ray system and quantitatively characterized in terms of statistical parameters associated with grayscale intensity probability distribution functions (PDF). Voidage data obtained by different measurement techniques are compared with time-average voidages obtained by X-rays and borescopic imaging. The hydrodynamic data provided by this study extend the benchmark database produced from the experiments performed on the TFB for validation of CFD and other models. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.