During various applications of gas fluidized units, the state/quality of the fluidized bed of solids can change due to changes in operation conditions or variations in the gas or solid feed. An analysis method is developed and applied which employs spectra of the fast Fourier transform of measured pressure fluctuations within the fluidized bed. The procedure is based upon the spectral analysis of successive time series of pressure fluctuations reflecting the actual hydrodynamic state (regime) of the bed. Practical application of the method is illustrated for fluidization at ambient temperature and pressure of spherical particles (glass beads) as well as irregularly shaped particles (limestone, sulfated limestone, and dried sewage sludge), in an 8-cm-ID fluidized bed. Using air, experiments were conducted with different fractions of the materials spanning a very wide range of 0.28-4.5 mm. The evolution was explored of bed behavior from the onset of fluidization to the pneumatic transport (elutriation) of solids from the bed. The method recognizes alterations of the hydrodynamic state of the bed caused by minor changes in the bed mass and/or in the granularity of the bed material. (C) 2008 American Institute of Chemical Engineers.