The phenomenon of temporary defluidization of stationary fluidized beds is investigated in this work. Throughout the development of a novel concept for the treatment of converter dust in laboratory experiments, the fluidizing gas was changed during the operational start up from inert gas (N-2) to reactive converter gas (CO, CO2 and N-2). At the same time, a decrease of the bed pressure drop and collapsing of the fluidized bed could be observed. The effect was temporary and after some time the initial fluidization conditions were fully restored. In this study, a systematic investigation is focused on the identification of possible mechanisms for the transitory defluidization and an evaluation of the influence of system parameters on the extent of the defluidization. The experimental work was carried out using two distinctly different systems. The converter dust application is run at elevated temperature of 400-600 degrees C and allows for the analysis of reactions and sintering effects. As the investigation of effects possibly caused by gas adsorption and changing gas properties at the high temperature system was too complex, investigations on these issues were done at ambient temperatures using a much simpler system. Results at ambient temperature show a strong dependence on adsorption effects and the difference in density of the fluidizing gases. The difference in density of both fluidizing gases causes the disturbance in fluidization due to different diffusion rates from the dense phase to the bubble phase. Additionally, at high temperatures, volume reduction due to chemical reaction and sintering effects have been identified as reason causing temporary defluidization. The decrease of voidage in the dense phase results in the collapse of the fluidized bed. Results obtained from our experiments confirm earlier investigations and validate the basis for further investigations in the area. Furthermore, specific parameters were developed for description of the intensity and the duration of the transient phenomenon. (c) 2005 Elsevier Ltd. All rights reserved.