Mixed matrix membranes (MMMs) incorporating conventional fillers frequently suffer from insufficient adhesion between the polymer matrix and the fillers. This often results in the formation of non-selective voids at the filler/polymer interface, which decreases the performance of the membrane. A novel approach is presented here to develop metal organic framework (MOF) based MMMs by using the self-assembly of MOF and polymer particles followed by their controlled fusion. MOF-polymer interaction is optimized through this strategy and it overcomes MOF-polymer incompatibility, MOF agglomeration and MOF. distribution problems, happening especially at high loadings of MOFs when applying conventional methods. Matrimid (R) polymer particles were first prepared by precipitating a Matrimid (R) polymer solution in wafer. The surface of these particles was then modified by the introduction of imidazole groups, enhancing the chemical compatibility with the selected ZIF-8 MOF. ZIF-8 nanoparticles were then grown in situ to this modified polymer particle suspension by addition of the precursor for ZIF-8 synthesis. The resulted suspension was cast to dryness and annealed in a solvent vapor environment to induce particle fusion, leading to a dense NIMM structure. Scanning electron microscopy (SEM) images showed an excellent dispersion of the ZIF-8 nanoparticles forming a percolating pathway without any agglomeration, even at 40 wt% loading of the ZIF-8. Excellent dispersion of ZIF-8 and an excellent ZIF-8-polymer interfacial adhesion resulted in a significant improvement in both CO2 permeability and CO2/CH4 selectivity. The CO2 permeability of the MMMs increased by 200% and the CO2/CH4 selectivity increased by 65% as compared to unfilled Matrimid (R). More detailed analysis of the gas transport performance of the MNIMs showed that the CO2 permeability and the CO2/CH4 selectivity are mainly governed by the increase in CO2 diffusivity. The presented approach is a very versatile MMM preparation route, not only for this specific ZIF and polymer but for a wide range of material combinations.