Abstract: Radial and axial mixing phenomena in a large three-phase fluidized bed can be described by the two-dimensional dispersion model. The model was analysed with a unsteady pointsource trace injection. The appropriate position for tracer measurement was determined according to simulation results. The tracer concentrations were measured by a special multichannel micro-probe. The radial and axial dispersion coefficients can be simultaneously estimated by using least-squares analysis in the time domain. In order to compare the, dispersion coefficients in different systems, the tracer experiments were done in a single liquid phase system, a liquid-solid two-phase system and a gas-liquid-solid three-phase system. In the single liquid phase system, both radial and axial dispersion coefficients were very small. In the liquid-solid two-phase fluidized beds, these dispersion coefficients were much higher than those in single liquid system. The radial dispersion coefficients of liquid in three-phase fluidized beds were greatly increased by the rising bubbles and moving particles.