Zeolitic-imidazolate framework ZIF-8 has shown great potential for high resolution separation of propylene/propane mixtures based on the size exclusion principle. Recently, we have successfully prepared highly propylene-permselective ZIF-8 membranes by developing a counter diffusion-based in situ method. As a follow-up study, here, we attempted to further improve the separation performance of ZIP-8 membranes prepared by the counter-diffusion method by controlling membrane microstructures via manipulating important synthesis variables as well as activation processes. Important synthesis variables investigated include the sodium formate to ligand ratios, the nature of zinc salts. With a sodium formate to ligand ratio of 0.12 and a mixture of zinc nitrate and zinc chloride, we have successfully synthesized ZIF-8 membranes with much improved propylene/propane separation performances with the separation factor of similar to 70 and the propylene permeance of similar to 268 x 10(-10) mol/m(2)s Pa. Furthermore, solvothermal activation processes were found to be effective in significantly increasing the propylene permeance while maintaining decent propylene/propane separation factors. (C) 2014 Elsevier Ltd. All rights reserved.