A unique phosphor tracer technique has been developed that measures the solids tracer concentration with time at different radial and axial positions within the cocurrent downflow circulating fluidized bed with a 140 mm inner diameter. Solids mixing is described by a proposed two-dimensional dispersed plug-flow model. The radial and axial dispersion coefficients under various operating conditions were determined from the experimental values of tracer concentration versus time. On the basis of these results, correlations with the bed voidage and Reynolds number are proposed.