The counter-diffusion method is an attractive approach for the preparation of zeolitic imidazolate framework-8 (ZIF-8) membranes for propylene/propane separation. In the present study, the effects of the preparation-solution concentration on the structure and the single-component and binary gas permeation properties of ZIF-8 membranes prepared on a porous a-alumina hollow capillary substrate were evaluated in order to explore the mechanism of membrane formation. ZIF-8 membranes were prepared using various concentrations of methanolic 2-methylimidazole and zinc nitrate solutions with a fixed molar ratio of 2-methylimidazole to zinc nitrate of 4. Increasing the solution concentration led to increased coverage of the ZIF-8 layer within the substrate, and ZIF-8 layers were eventually generated even on the inner surface of the substrate. A higher solution concentration facilitated inter-diffusion of the solutes in the counter-diffusion method. Binary permeation measurements using an equimolar mixture of propylene and propane subjected to a total pressure difference of 0.1 MPa at 298 K gave rise to a propylene permeance of 7.0 x 10(-9) mol.m(-2).s(-1).Pa-1 with a maximum achieved propylene/propane selectivity of 42.