Fluidized beds employing fine powders are finding increased application in the chemical and petrochemical industry because of their excellent mass and heat transfer characteristics. However, in fluidized bed chemical reactors axial gas back mixing can strongly decrease the conversion and selectivity. By insertion of membranes in fluidized beds large improvements in conversion and selectivity can be achieved, firstly by optimizing axial concentration profiles via distributive feeding of one of the reactants or selective withdrawal of one of the products, and secondly, by decreasing the effective axial dispersion via compartmentalization of the fluidized bed. Moreover, insertion of membrane bundles in a suitable configuration impedes bubble growth, thereby reducing reactant by-pass via rapidly rising large bubbles. In this work the influence of the presence and configuration of membrane bundles and the effect of gas addition via the membranes on the effective axial dispersion was studied experimentally. Steady state concentration profiles were measured where a CO2-tracer was injected at different locations through a probe (point injection) or via the membranes (line injection) into a square fluidized bed View the MathML source containing glass particles (75–View the MathML source, 2550 kg/m3) fluidized with nitrogen distributed via a porous plate. Different bed configurations, viz. without internals, with vertical or horizontal membrane bundles were investigated and the effects of overall fluidization velocity and gas flow ratio of gas fed through the membrane bundles and the porous plate distributor were studied. Experimental results revealed that the insertion of vertical and horizontal membrane bundles decreases the effective axial dispersion considerably compared to a bed without internals. The point injection experiments indicated the importance of a non-uniform lateral emulsion phase velocity profile. The line injection experiments clearly pointed out the importance of bubble-to-emulsion phase mass transfer limitations. Gas addition through the membrane bundles decreases the effective axial gas dispersion enormously by almost annihilating the solids down flow along the walls and by decreasing the average bubble size and bubble fraction.