Bed collapse experiments have been carried out in fluidized beds of agglomerated nano-powders possessing high void fractions. Transient responses for the local pressure-drop as well as the gas velocity during the collapse were recorded. From the local pressure transient data, occurrences of key events of the collapse dynamics were identified. The first event is the zero pressure-drop condition soon after the flow interruption while the second is the global minimum on the pressure-drop profile that marks the end of the first stage of the bed collapse characterized by fast bed transients. The second stage is a relatively slow process, which ends with the onset of the zero pressure-drop condition again (third event) that reflects particles becoming stationary, thus signaling the completion of the collapse dynamics. The time of the first stage of the collapse is found to vary linearly with the initial fluidization velocity at which the collapse experiment was initiated whereas the total collapse time shows a rather weak dependence on the fluidization velocity. On the other hand, the velocity transients during the collapse follow first-order time lag that can be accurately modeled using a single time constant irrespective of the initial fluidization velocity. (C) 2013 The Society of Powder Technology Japan. Published by Elsevier B. V. and The Society of Powder Technology Japan. All rights reserved.