The residence time distribution of solids in an 18-1 pilot scale three-phase annular reactor has been obtained as a function of gas and liquid flow rates in order to characterize the influence of fluid dynamics on solids mixing. Both co-current and countercurrent operations were investigated. Gas velocity in the range 0.05-0.5 cm s(-1) was employed, while the liquid velocity was varied between 0.03 and 0.13 cm s(-1). Commercial titania powder was used as a tracer. The RTD curves for the co-current operation showed a bimodal behavior whose peaks decreased with decreasing gas flow rate. As a result, the solid flow dynamics was modeled by a series of CSTRs in parallel with a plug flow reactor. The model also admitted interphase solid transport between the wake and bulk liquid phase and yielded predictions with a mean squared error between 3% and 6%. The RTD behavior for countercurrent operation was, however, unimodal and therefore modeled as a series of stirred tanks with a recycle stream. The associated mean squared error was in the range 1-4%. Although fluid flow rates were held constant during tracer experiments, there seemed to be an evolution of solids flow pattern within the reactor as evidenced by the time-dependency of the intensity function lambda.