Experiments involving monodisperse Geldart Group B particles have been carried out in a pilot-scale riser of a circulating fluidized bed (CFB). Several combinations of superficial gas velocity (U-s), solid flux (G(s)), average particle diameter (d(ave)), and particle material density (rho(s)) were investigated. Surprisingly, the experiments reveal the presence of a reverse core-annulus profile (i.e., a dense core with a dilute annulus) under certain conditions. Specifically, for the large glass beads (d(ave) = 650 mu m, rho(s) = 2500 kg/m(3)), the reverse coreannulus profile was observed near the top of the riser for all U-s and G(s) combinations examined. For high-density polyethylene (HDPE) pellets (d(ave) = 650 mu m, rho(s)=900 kg/m(3)) of the same d(ave), reverse coreannulus was observed at the top of the riser only at relatively low G(s). However, for the smaller glass beads (d(ave)= 170 mu m, rho(s)= 2500 kg/m(3)), the traditional core-annulus profile was observed for all U-s and G(s) combinations. Although previous work provides possible explanations for this behavior (gas-phase turbulence, etc.), the evidence obtained in this system suggests a novel dominant factor for reverse core-annulus flow: the particle Stokes number (St). Lower-St particles are more apt to follow the gas exiting the riser, while higher-St particles have a longer relaxation time and thus are more likely to re-enter the riser after collision with the roughened rounded-elbow exit. Accordingly, the re-direction of particles from the rounded-elbow exit and back into riser due to large-scale roughness along the elbow is greater for higher-St particles. (C) 2011 Elsevier B.V. All rights reserved.