For ultra-pure hydrogen production packed-bed and fluidized bed membrane reactor concepts have been proposed. Due to the development of novel ultra-thin Pd-membranes with higher selective hydrogen permeabilities, packed-bed membrane reactors suffer from inefficiencies related to concentration polarization and heat management. These problems can largely be overcome in fluidized bed membrane reactors. The focus of this work is the better understanding of the hydrodynamics in the vicinities of membranes immersed in fluidized beds. In this study, it has been proven that a new effect might be detrimental for the performance of a fluidized bed membrane reactor: the formation of bubbles, or better 'gas pockets', surrounding (and attached to) the membranes. A non-invasive PIV-DIA technique has been employed to study the gas pockets behavior in detail in a pseudo 2D column with horizontally immersed membranes. Experimental results have confirmed that gas pockets are free of solids, do not rise like bubbles but stay attached to the membranes thus reducing the effective contact area with the emulsion phase, have very short life times and appear frequently in the bed. A gas pocket is formed with gas from the emulsion phase and thus no gas exchange takes place between this gas pocket and the surroundings, implying an important mass transfer limitation. Due to these characteristics, unlike previously done in literature gas pockets should not be considered as bubbles when studying hydrodynamics. A new method, able to distinguish between a gas pocket and a bubble, has been developed with accuracy over 95%. The effect of different experimental conditions on gas pockets is also reported. Furthermore, a simulation study has been performed with a Two-Fluid Model including the Kinetic Theory of Granular Flow (KTGF) that has shown good agreement with the experimental observations and has provided a more detailed insight into the formation of the gas pockets. For a similar configuration, average bubble diameter after gas pocket removal from the analysis is 2.4 and 2.5 cm for experimental and simulated results. Moreover, gas pockets characteristics are in a good agreement, being 1.3 and 1.1 cm on average for experimental and TFM simulations, respectively. Finally the mechanism of formation of a gas pocket and its influence on the behavior of a fluidized bed membrane reactor are discussed in detail. (C) 2015 Elsevier B.V. All rights reserved.