One bed of glass ballotini and several beds of potato starch were fluidized with air at varying fluidization velocities, while the distributor plate was being vibrated at different amplitudes and frequencies to study the effect on the fluidization quality. The degree of cohesiveness of the potato starch powders was altered by varying the moisture content. The cohesiveness was measured in a triaxial cell at different moisture contents and values of the unconfined yield strength are reported as functions of the preconsolidation Stress. Results of the quality of fluidization are presented mainly in the form of three-dimensional plots showing the variation of the fluidization index as a function of the superficial fluidization velocity and a dimensionless vibration intensity number. Critical values of this vibration number and the fluidization velocity, beyond which the powders would fluidize well, were determined under the conditions investigated. The vibration intensity required to fluidize the powders well increased with increasing cohesiveness of the powder, as did, to a lesser extent, the required superficial gas velocity. The parameters affecting the quality of fluidization are discussed.