A thick continuous zeolitic imidazolate framework-8 (ZIF-8) membrane is crystallized on a highly porous flexible polymer (polysulfone) surface by in-situ synthesis. Micro and nanosized metal organic framework (MOF) particles are combined to avoid holes or defects in the membranes, which are characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-gravimetric analyses (TGA) and N-2 sorption analysis. ZIF-8 content of the membrane exceeds 75 wt%, indicating an almost self-supported MOF-layer membrane. Permeation tests show high H-2/CH4 (10.5 +/- 0.6) and H-2/N-2 (12.4 +/- 0.9) selectivities in separation of gas mixtures, confirming the expected molecular sieving effect due to MOF microporosity. These selectivity values are the highest among those reported for polymer-supported ZIF-8 membranes. Finally, in the H-2/CH4 mixture the measurements carried out as a function of temperature (35-80 degrees C) show activated transport. (C) 2014 Elsevier B.V. All rights reserved.