Mixing in monosized particles and binary mixtures of solids in three-phase fluidized beds is investigated by means of a non-invasive Radioactive Particle Tracking technique (RPT). Pulses of particles at different column heights are constructed from the trajectory of a single radioactive tracer whose motion is tracked for several hours. For each pulse released, number distributions of particles in the axial direction are thus obtained for each instant of time from injections at different axial positions in the reactor. Using this information, axial mixing times for tile solids are measured for the experimental conditions studied. A one-dimensional two-zone model based on the three-phase counter-current backmixing model used for gas-solid fluidization (Gwyn er al., 1970) is proposed and solved for these conditions to calculate solids axial number distributions and mixing times. Agreement between experimental and predicted results is satisfactory.