The potential of fluidized beds for conducting intrinsic kinetics studies of fast and highly exothermic reactions was investigated via gas residence time (RTD) experiments in the fixed and bubbling bed regime with vanadyl pyrophosphate catalyst (VPO) containing 10%, 20%, 30% and 40% fines (catalyst fraction with d(p) < 44 mu m). In order to characterize the divergence from plug flow, the RTD data was compared with two simple hydrodynamic models: axial dispersion and n-CSTRs in series. Results show that the simple models characterized well the gas phase hydrodynamics with the deviation from plug flow increasing with gas velocity. At velocities higher than minimum bubbling, the RTD curve changes from an ideal Gaussian shape and develops a "shoulder" or "tail", which represents a deviation from ideal flow. It was found that fines had no effect upon the axial dispersion and perforated plate internals can decrease axial gas dispersion for the experimental conditions used. The results suggest that fluidized bed may be an alternative to fixed bed reactors for kinetic studies at low velocities and that internals may reduce the deviations from plug flow in the bubbling regime. Moreover results show that a sudden decrease in Peclet number (Pe) characterizes the initiation of the bubbling regime. Based on this, a novel method for the experimental measurement of U-mb is proposed. (c) 2006 Elsevier B.V. All rights reserved.