The trajectories of a solid tracer with the same characteristics as the circulating solids in the risers of two pilot-scale circulating fluidized beds (CFBs), obtained by the computer automated radioactive particle tracking (CARPT) technique, are examined in detail to get further insights into the complex solids dynamics of these systems. The analysis uses tools from the theory of non-linear dynamics and symbolic dynamics. Distinct features of the solids dynamics within the fast fluidization and the dilute phase transport flow regimes are highlighted and related to the operating conditions. The possibility of downflow cluster existence within the central region of the riser is particularly examined. (c) 2006 Elsevier Ltd. All rights reserved.