Solid-liquid stirred tank reactors are widely used in the chemical, biochemical and mineral processing industries. Characterization of solid hydrodynamics in these reactors is essential to optimize the design and operation of such reactors. Although few studies exist of the solid hydrodynamics in dilute suspensions, a systematic investigation with relatively dense systems is still lacking in the open literature. The noninvasive computer automated radioactive particle tracking (CARPT.) technique is used in the current work to study the solids flow field in dense solid-liquid suspensions (2.5-19% solids loading) in a stirred tank. CARPT provides Lagrangian description of the solids flow field which is used to obtain the time-averaged velocity fields and the turbulent quantities. The Lagrangian information has also been used to calculate the solids sojourn time distribution at different axial regions in the reactor, which clearly show that the Zwietering's correlation over-predicts the "just-suspension" speed at which incipient suspension of solids is achieved. (c) 2007 Published by Elsevier Ltd.