The use of free surface fluctuations in three-dimensional (3-D) fluidized bed dynamic characterization is addressed. This technique provides a noninvasive global measure of the quality of fluidization by measuring at one position in the bed. The well-known effect of particle size on fluidized bed hydrodynamics was explored using a 3D fluidized bed operated with four different particle groups. The analytical methods used were mutual information junctions, frequency, and state space analysis of time series collected from bed surface fluctuations. Turbulent, multiple, and noninteracting bubble flow patterns, occurring during bubble regimes, have been characterized. Kolmogorov entropy for the reconstructed attractor is very sensitive to the transition between fluidization regimes. Moreover, correlation dimension estimates ranging between 3 < D-2 < 5 indicate apparent low-dimensional dynamics. Additionally for larger particle groups, D-2 reaches a value close to 3, indicating slow bubbling controlled by large bubbles before the transition between difterent bubble flow patterns takes place. Finally, particle size has been found to have strong influence on signal persistence. (C) 2004 American Institute of Chemical Engineers.