The effects of the mass ratio of binary solids with fine and coarse particles on solid mixing and segregation were investigated in a gas-solid circulating fluidised bed with 0.1-m inner diameter and 3.7-m height. A sampling valve at the bottom of the bed was used to confirm the composition of the bed, and an extraction probe at the top of the bed was used to distinguish between the upward and downward flows. Iron ore (41-mu m average particle size) of Geldart group A was used for the fine particles, and iron ore (130-mu m average particle size) of Geldart group B was used for the large particles. The fluidisation regions of the riser were classified according to differential pressure into a dense bottom zone ((epsilon) over bar (s)>0.3), a dense transition zone (0.01 (epsilon) over bar (s)<0.3), and a dilute zone (<(epsilon)over bar>(s)<0.01). In the dilute zone, a typical core-annulus flow structure appeared, while the mixing of coarse particles positively correlated with the mass ratio of fine particles to the total amount of particles in the dense phase zone. Coarse particles were not easily entrained, and several entrained particles re-circulated through the downflow. This recirculation affected the dense phase flow. (C) 2014 Published by Elsevier B.V.