In three case studies, (i) 1-butene/i-butene separation on a ZSM-5 membrane, (ii) carbon dioxide/methane, and (iii) ethene/ethane separations on a zeolitic imidazolate framework 8 (ZIF-8) type metal organic framework (MOF) membrane, it is demonstrated that the membrane selectivity for the separation of a gas mixtures can be well described and even predicted by the interplay of mixture adsorption and mixture diffusion according to the simple relation membrane selectivity = adsorption selectivity x diffusion selectivity. The mixed gas adsorption and diffusion data which are necessary for the above calculation can be derived cost and time effective by sorption uptake/desorption studies on giant single crystals with IR microscopy (IRM) detection.