This paper presents an experimental and computational study of the flow behavior in a cold flow laboratory scale circulating fluidized bed reactor. Laser Doppler Anemometry and Phase Doppler Anemometry techniques are used to measure mean and fluctuating velocity, diameter and solid concentration, simultaneously. An axial segregation by size and its variation with the superficial gas velocity are demonstrated. Also, a significant radial segregation is measured in the riser. A multi-fluid Computational Fluid Dynamics model has been developed and verified against the experimental results. The flow model is based on an Eulerian description of the phases where the kinetic theory for granular flow forms the basis for the turbulence modeling in the solid phases. The model is generalized for one gas phase and N number of solid phases to enable a realistic description of the particle size distributions in gas/solid flow systems. Each solid phase is characterized by a diameter, form factor, density and restitution coefficient. The computational results agree well with the measurements. (C) 2000 Elsevier Science Ltd. All rights reserved.