In a fluidized bed, particle agglomeration and channeling have been observed when the fluidizing gas is switched from lower to higher density. This defluidization is a transient phenomenon, and fluidization is restored after several minutes. In this paper, we assume this behavior can be explained by non-equimolar diffusion, which causes pressure gradients in the bed and subsequently leads to viscous flow. To verify this assumption, pressure changes were measured for several binary diffusion systems in a packed bed. After the packed bed was filled with a gas, another gas was supplied to the bed's outer surface to replace the first gas. These gases were exchanged by diffusion, and the pressure at the closed end of the column either increased or decreased due to differences in the gases' molecular weights. The pressure changes were compared with pressure changes calculated from a model based on both an unsteady-state diffusion equation and the Kozeny-Carman equation. The measured pressure changes in the packed beds could be correlated with the calculated values without any adjustable parameters. In a fluidized bed, after the gas switched from lower to higher density, differences in diffusion rates due to the gases' various molecular weights caused a net molecular flow from the emulsion phase to the bubble phase. This flow caused a decrease in gas velocity in the emulsion phase and a contraction of the emulsion phase; channeling and defluidization were then observed.