A small laboratory-scale membrane-assisted fluidized bed reactor (MAFBR) was constructed in order to experimentally demonstrate the benefits of this reactor concept, especially the enhanced bubble-to-emulsion phase mass transfer and the reduced overall axial gas phase back-mixing, due to the presence of the membranes and permeation of gas through the membranes. With steady-state tracer gas injection experiments, it was demonstrated that the experimental reactor exhibited approximately plug flow behavior for all the operating conditions investigated in this work as a result of the elimination of macroscale circulation patterns due to the presence of the membranes and, even more importantly, the permeation of gas through the membranes. With an ultrasound technique, the gas residence time distribution (RTD) of the MAFBR was measured over a wide range of fluidization velocities for two different bed heights. Interpretation of the RTD measurements with a phenomenological two-phase reactor model extending the bubble assemblage model proposed by Kato and Wen (Kato, K.; Wen, C. Chem. Eng. Sci. 1969, 24, 1351) showed that the average bubble diameter is significantly decreased for higher ratios of gas permeated through the membranes relative to the total gas flow rate.