This study provides a straightforward approach towards highly diffusion-selective mixed matrix membranes (MMMs) to upgrade He from CH 4 . In summary, we have developed a modulated synthesized approach, yielding druse-type hollow crystals of MIL-116(Ga), a so called "dense" metal organic framework (MOF). MIL-116(Ga) is non-porous towards gases such as CO 2 and N 2 due to crystal water in its structure, as shown in its crystal structure and in sorption measurements. However, it offers the possibility for diffusive permeation of He with the kinetic diameter d kin = 2.6 Å. The free pathway inside the crystal druse allows to speed up diffusion, while completely reflecting CH 4 at its surface, making MIL-116(Ga)-formate an ideal molecular sieve. Through incorporation of the druse-like MIL- 116(Ga)-formate crystals into a polysulfone (PSU) matrix, an inexpensive and diffusion limiting polymer, high performance MMMs were made. From microstructural investigations, an ideal polymerfiller interaction is observed, attributed to H-bonding between -COOH groups of the MOF and PSU.