Solids movement in a gas-solid fluidized bed was monitored by a Radioactive Particle Tracking (RPT) technique. The gas was air at room temperature and atmospheric pressure and the solid was sand or FCC powder. The experiments were done at superficial gas velocities from 0.5 to 2.8 m s(-1) for sand, and 0.44 to 0.9 m s(-1) for FCC, covering both bubbling and turbulent regimes. A variety of solids behaviours, including restricted axial movement and unrestricted axial movement of solids, were studied by processing the data in an appropriate manner. The variance test was performed on the Residence Time Distribution (RTD) of solids in the restricted upward and downward movement. The results of the test clearly showed that upward movement of solids could be explained by a diffusive mechanism, while the mechanism of downward movement of solids should be considered as convective. The variance test for unrestricted axial movement of solids showed that this type of movement cannot be explained by either of the two above mentioned mechanisms. In order to model the axial movement of the solids, it was assumed that solids move according to the two diffusive and convective mechanisms. By comparing the RTD of unrestricted movement of solids with the unsteady state formulation of the solid movement, it was found that the solid velocity in the convective term of the model have to be less than the velocity of a single particle moving in a fluidized bed at the same condition. This fact suggests that the solid particles do not move separately in the fluidized bed, but move as part of a solid aggregate, or cluster, which exercises a drag force greater than that of a single particle and, therefore, moves at a lower velocity.