The hydrodynamics of a circulating fluidized bed (CFB) were studied using radioactive argon as a tracer. The impulse experiments show that gas passes through the riser at velocities significantly greater than superficial gas velocities. A core-annular flow model was used to describe the hydrodynamics. Axial dispersion in the core zone is negligible and mass transfer to the denser, stagnant annulus is characterized by a cross-flow coefficient, k. The Gilliland-Sherwood equation for wetted wall towers, modified to account for the increase in mass transfer rate introduced by the solids, correlates the data reasonably well: ▪ The ratio of the cross-sectional area of the lean core to riser cross-sectional area, Φ g , decreases with solids mass flux but increases with gas velocity. Based on a number of experimental investigations, the effects of riser diameter and particle characteristics on Φ g were qualified and the data were fit with a simple two parameter model: ▪ This relationship adequately predicts gas bypassing in risers from 0.05 to 0.94 m in diameter.