Approaches have been made to combine ZIF-8 (Zeolitic Imidazolate Framework 8) and fullerenes, with one of the best results for He-upgrading so far, reaching a selectivity α(He/CH 4 ) = 9.2 with a permeance of 210 GPU at only 3.5 wt.% loading of C 70 . Thus far, MOF materials could not show to be better than existing carbon membranes. Using MOFs in polymers for He-upgrading, such as HKUST-1 (Cu 3 BTC 2 with BTC 3-=1,3,5-Benzenetricarboxylate) in Matrimid ® (a polyimide), has shown highly promising results. Loading Matrimide ® membranes with 40 wt.% HKUST offered a selectivity of α(He/CH 4 ) = 392 with a He-permeability of P = 64.3 Barrer , but also these membranes performed only slightly better than the benchmark from polymers, gathered by Robeson . Many different synthetical approaches have been made to eliminate lattice defects and improve molecular selectivity. Nevertheless, the intrinsic flexibility of MOFs has always been a key issue, and hence much research was invested to improve the porosity and tune the adsorptive features of MOFs in polymer-filler MMMs. The polymer-filler combination is a crucial factor that must be taken into account during the design of the MMMs. We believe to have identified metal-organic frameworks as suitable molecular sieves starting from our recent study on gas diffusion in ZIF-62 glasses, which lost a considerable amount of its porosity upon melting and thereby became impenetrable for larger gases. However, tiny gas species were still able to diffuse rapidly through these MOF-glasses, offering very precise molecular sieving. This lead to the idea to revisit MOFs as molecular sieves which were until now considered as "dense" or "nonporous". The crystalline metal-organic framework MIL-116 (Matériaux de l′Institut Lavoisier 116) is among the materials considered as dense and non-porous, and seems based on its crystallographic features (i.e. void volume of the structure) suitable for He-sieving. The term "non-porous" originates from the disability of these MOFs to take up N 2 (d kin = 3.6 Å) or Ar (d kin = 3.43 Å) gas, which also was the common opinion on ZIF-62 (and is the current opinion on MIL-116 ). In MIL-116, non-removable crystal water is contained in the pore space, rendering the pores extremely narrow and thereby highly suitable for precise molecular sieving of He from CH 4 . To optimize polymer-filler interaction, we follow another concept for MMMs synthesis, which is the marriage of MOFs with polymers of the same diffusivity. The solution-diffusion model as the underlying transport mechanism in polymers is highly depending on the ordering and eventual crystallinity of the polymeric chains. To reduce the impact of adsorption in the MOF-filler and the diffusion barrier, the diffusive transport through the MOF must also be slow. When a polymer has a high density and shows very slow permeability, the MOF should also provide narrow pores and slow diffusion, while still able to enhance gas permeability of the matrix. In this study we decided for the thermoplastic high-performance polymer polysulfone (PSU), which has a very low permeability, and married it with MIL-116(Ga). Prior studies have reported significant performance for He/CH 4 and H 2 /CH 4 separation with MMMs using highly fluorinated polymers . Nevertheless, due to rising awareness of the toxicity and longevity of per-and polyfluorinated substances (PFAS)in the environment, which comes mainly from polymer chemistry, the use of simple polymers is very favourable and investigated in this study for its feasibility.