A hydrodynamic model describing the flow structure of fluid catalytic cracking particles in laboratory scale downflow reactors is proposed. A correlation, incorporating the operating conditions of solids circulation rate and superficial gas velocity has been developed which adequately correlates published data of solids hold-up for fully developed flow. From this, the radial gas and particle velocity profiles are determined. Mechanisms for the densification near the wall in downflow reactors are proposed in order to describe changes in the radial solids hold-up profiles with changing operating conditions. The operating conditions of superficial gas velocity and solids flux are related to the magnitude of the densification near the wall in terms of an energy minimization concept. The flatness of the solids hold-up profile can be described by the model parameter a. A low a indicates a nat solids hold-up profile, more homogeneous flow and better solids-gas contact efficiency. A slip velocity is calculated using the model which confirms that segregation increases with decreasing superficial gas velocity and increasing solids flux. Thus, the model is able to describe the radial flow structure and suspension homogeneity according to operating conditions and the importance of friction between the gas-solids suspension and the wall for laboratory scale downflow reactors. The study also highlights problems in using the hydrodynamic data obtained in laboratory scale downflow reactors for scale-up, since wall effects are likely less important in industrial scale downers. Different reaction schemes (both series and parallel) have been employed to study the effect of the operating conditions on conversions, yields and selectivities in the presence of fast deactivating catalysts. The study highlights the importance of using realistic hydrodynamic models to interpret kinetic data obtained using laboratory scale downflow reactors.