The dispersion of pulses of a range of shapes through a bubbling fluidized bed has been studied using an ECAT EXACT HR+ PET camera. The equipment and materials used made it possible to label actual bed particles with a radioactive labeler for use as tracer. The experimental equipment and procedure as well as the data analysis procedure are described. The results are shown as bitmaps, 3-D plots, and contour plots of the relative tracer concentration during 1-s time intervals. The particle pulse is shown to exhibit a cyclical axial motion while it is being dispersed. The results are compared with a phenomenological stochastic model for particle dynamics. It is further shown that the model, which is based on the notions of upward material transport with the fluidization bubbles and material descent with dispersion in the bulk, qualitatively captures all the essential features. Material transport only in bubble wakes, however, could not account quantitatively for the results. The discrepancy is likely attributable to "gulf streaming" in the bed, a phenomenon that should be studied experimentally and accounted for in models for particle dynamics in fluidized beds. (C) 2005 American Institute of Chemical Engineers.