In this work, an investigation on the fluidization behaviour of solid particles was carried out while the fluidizing agent was switched from one gas to another with an equal volumetric flow. Four different gases, namely helium, nitrogen, argon and carbon dioxide (He, N-2, Ar and CO2), were adopted as fluidizing agents, and fluid catalytic cracking (FCC) was used as the experimental material. When changing the gas supply a sharp decrease of the bed pressure drop and significant compression of the emulsion phase are observed, as well as the disappearing of the bubbles and the formation of channels inside the bed. However, these disturbances are temporary, and the initial fluidization conditions can be restored after a certain amount of time. Such behaviour is quantitatively described by measuring the maximum relative decrease of bed pressure drop, the time duration and the intensity of the defluidization phase. The effects of the initial powder bed height, the switching gas velocity and the gas properties are analyzed as well. Finally, a semi-empirical model was developed to predict the defluidization time based on the understanding of this physical process and analyzing the experimental data. (C) 2020 Elsevier B.V. All rights reserved.