Diffusivity of the solid particles in a 152-mm ID gas-solid fluidized bed was determined at different regimes of fluidization. The gas was air at room temperature and atmospheric pressure and the solids were 385 mum sand or 70 mum FCC particles. The experiments were done at superficial gas velocities from 0.5 to 2.8 m/s for sand and 0.44 to 0.9 m/s for FCC tin both bubbling and turbulent regimes). Movement of a tracer was monitored by radioactive particle tracking (RPT) technique. Once the time-position data became available, local axial and radial diffusivity of solids were calculated from these data. Calculated diffusivities are in the range of 3.3 x 10(-3) to 5.6 x 10(-2) m(2)/s for axial and 2.6 x 10(-4) to 1.5 x 10(-3) m(2)/s for radial direction. The results show that the diffusivities, both axial and radial, increase with superficial gas velocity and are linearly correlated to the axial solid velocity gradient. Solid diffusivity in a bed of FCC was found to be higher than that of a bed of sand at the same excess superficial gas velocity.