Mixing and circulation of monosized particles in laboratory-scale tapered spouted beds have been characterized experimentally by measuring non-invasively the 3-D trajectory of a single tracer via a radioactive velocimetry technique. Processing the obtained Lagrangian trajectory allowed determination of the mixing dynamics in the longitudinal, radial and circumferential directions, of the return length and return time distributions, and of the mean Eulerian flow fields. A conceptual solids flow structure has been delineated. A four-zone 2-D axisymmetrical Monte Carlo model has been developed for emulating the elementary steps in play in the longitudinal mixing, i.e., the direction of slowest mixing, and in the return (or circulation) time and length of the solids phase. The four-zone solids flow structure is viewed as: (i) a spout region with a constant upward particle velocity, (ii) an annulus region above the conical base with a downward velocity radial profile, (iii) an annulus region within the conical base where the linear velocity, considered to be parallel to the cone wall, is equal to that of the incoming particles, (iv) a fountain in which the particle movement is characterized by the particle residence time, an exiting radius, and an average fountain height. The model proved successful in restoring the measured return time and return length distributions, and the mixing response curves. (C) 2003 Elsevier Science Ltd. All rights reserved.