Fluidized Bed Membrane Reactors gain worldwide increasing interest for various applications. Nevertheless, fundamental research on the hydrodynamics of these reactors is required in order to improve the predictive capabilities of numerical models and to improve reactor performance. This study comprises an experimental investigation in a pseudo-2D gas-solid fluidized bed containing horizontal submerged membrane tubes, through which gas can be added to - or extracted from - the fluidized suspension. Employing Particle Image Velocimetry (PIV) combined with a novel Digital Image Analysis (DIA) technique, the effect of the presence of, and permeation of gas through the membranes, on both the solids flux profiles as well as the bubble properties has been investigated in great detail for different membrane configurations. It has been shown that merely the presence of the membrane tubes decreases the solid fluxes by about a factor three, but does not significantly change the overall circulation patterns in the bed. The permeation ratio and the membrane tube arrangement (in-line vs. staggered, size and number of membranes) hardly influence the time-averaged solid flux profiles. In addition, it has been found that, compared to a fluidized bed without inserts, the average bubble size with horizontally submerged membrane tubes decreases by a factor of approximately three, but is hardly dependent on the permeation ratio. The average total superficial gas velocity is more important than the permeation ratio. With respect to bubble size and number of bubbles, the staggered membrane arrangement with 9.6 mm tubes outperforms the in-line arrangement, the arrangement with smaller tubes and the arrangement with fewer membrane tubes, which is of great benefit to reduce bubble-to-emulsion phase mass transfer limitations in fluidized bed membrane reactors under reactive conditions. (C) 2013 Elsevier B.V. All rights reserved.