The residence time distribution (RTD) of solids is essential for the design of CFB reactors where conversion proceeds with time. The residence time distribution for the solids is measured at different working conditions. The resulting average residence times are correlated as function of gas velocity and solids circulation rate and are compared with literature data. In order to predict the RTD of the solids, the solids/gas flow is firstly described by a plug flow with dispersion. A fitting procedure gave experimental Peclet numbers, which were correlated as function of gas velocity and solids circulation rate. A more fundamental approach based on the core/annulus flow structure is thereafter used to predict the residence time. The riser is divided in a dilute core with particles flowing upward and a denser film moving slowly downward along the wall. Particle interchange between the two regions is described by a convective interchange flow. The model is used to predict experimental RTD-curves. The interchange flux between core and annulus corresponds well with radial fluxes reported in literature. Although the RTD is qualitatively well described, experimental curves show a higher dispersion than the calculated ones. To improve the core/annulus approach, further research is necessary.