The newly developed MOF membrane ZIF-8 separates an equimolar ethene/ethane mixture at room temperature for 1 and 6 bar feed pressure, respectively, with a selectivity of 2.8 and 2.4. Independent sorption uptake studies of an ethene/ethane mixture on a big ZIF-8 single crystal by IR microscopy detection show in combination with grand canonical Monte Carlo simulations that this moderate ethene selectivity of the ZIF-8 membrane can be explained by the interplay of a preferential ethane adsorption selectivity competing with a preferential ethene diffusion selectivity. This means, that ethane adsorbs stronger than ethene, but ethene diffuses faster and overcompensates the adsorption preference of ethane, resulting in a membrane permeation selectivity for ethene. (C) 2010 Elsevier B.V. All rights reserved.