The time-averaged solids velocity profiles in a freely bubbling, three dimensional, gas-solid fluidized bed have been experimentally quantified for two different bed materials and different bed aspect ratios at different superficial gas velocities by performing positron emission particle tracking (PEPT) experiments. A cylindrical fluidized bed with an inner diameter of 0.306 m was filled with either glass beads with a diameter of 400-600 mu m or with linear low density polyethylene (LLDPE) particles with a diameter of 1000-1300 mu m, having approximately the same ratio of Ar/Re-mf, trying to realize dynamic similarity. Fluidization of both bed materials showed Geldart B type behavior. At lower superficial gas velocities two distinct vortices appear above each other for both types of bed material; when the superficial gas velocity is increased, the lower vortex disappears and the top vortex spans the entire height of the bed. Although qualitatively the same phenomena were observed for both bed materials, the time-averaged solids phase circulation rate in the fluidized bed filled with LLDPE particles was significantly higher than the time-averaged solids phase velocity in the fluidized bed filled with glass beads, despite the similar ratio of Ar/Re-mf. When the bed aspect ratio is increased from 1 to 1.5, the vortices become elongated without altering the solids circulation rate. Differences in the particle-particle collisional properties (coefficients of restitution and particle friction coefficients) may be the cause of the observed quantitative differences in the bed hydrodynamics via their influence on the bubble characteristics. (C) 2012 Elsevier B.V. All rights reserved.