Membrane-assisted micro-structured fluidized beds have been proposed as efficient membrane reactors and in particular for hydrogen production. However, very little information is available on the effects of gas permeation through the membrane on the gas back-mixing and mass transfer rates in this type of reactors. This study carefully investigates the gas back-mixing characteristics in a micro-fluidized bed and the influence of gas permeation (both addition and extraction) through flat membranes installed in the left and right walls confining the fluidized bed. The tracer gas injection (stimulus-response) method has been used combined with Particle Image Velocimetry and Digital Image Analysis techniques for additional insightful information on the solids flux profiles that strongly influence the gas back-mixing. The results of this study demonstrate that micro-structuring of the fluidized bed results in reduced gas back-mixing and thus in a more efficient membrane reactor configuration for hydrogen production because of the increased driving force for gas permeation. The results also provide some guidelines for the design of micro-structured fluidized bed membrane reactors in terms of preferred specific membrane area and particle size. (C) 2014 Elsevier Ltd. All rights reserved.