When the gas flow to a fluidized bed is suddenly interrupted, the pressure drop variation with time follows a logistic dose response function. The time it takes the bed to deaerate, or collapse, is a function of system geometry, particle characteristics and the fluid properties. For certain Geldart Group A powders, the collapse time may double by increasing the fines (d(p)<44 μm) particle fraction from 10% to 40%. It is almost independent of gas velocity between U-mf (minimum fluidization velocity) and up to 20 times U-mf, and it increases with gas viscosity. We characterized the deaeration rate assuming the bed is at incipient fluidization after the gas flow is interrupted and the pressure drop follows the Kozeny-Carman equation. The model predicts the bed expansion and U-mf. (C) 2003 Elsevier Science B.V. All rights reserved.