A predictive mathematical model, able to characterize and quantify all facets of the time-averaged gas and solids flow structure and properties within circulating fluidized bed (CFB) risers, is proposed. The model can be used as a tool to assess a-priori the operation of a riser and can be easily coupled to kinetic models for process simulation. The input parameters to the model include the riser operating conditions (that is, solids circulation flux and gas superficial velocity), riser geometry and gas and solids physical properties. The proposed model assumes the CFB riser to be axially composed of two regions : an acceleration zone at the riser base, where solids re-injected from a standpipe are accelerated to a constant upward velocity, and a fully-developed region, where the flow characteristics are invariant with height, extending from the end of the acceleration region to the riser exit. The model postulates the existence of a core-annulus type of flow structure and is based on both fundamental principles and empirical relationships. The model is successfully verified against experimental data from CFB units of various sizes and operating under different regimes of fluidization. The model outputs, consisting of axial pressure drop profiles, axial and radial voidage profiles, radial solids velocity and mass flux profiles, average gas velocity and core radius, are compared to existing data and are assessed critically.