In this work, the effectiveness of metal-organic frameworks (MOFs) with sheet-like morphologies was assessed as function of the MOF microporosity and MOF-matrix compatibility. Zeolitic imidazolate frameworks (ZIFs, a MOF subclass) with sheet-like/platelet morphologies were incorporated in Matrimid/PBI matrices resulting in mixed matrix membranes (MMMs). The ZIFs were either permeable (ZIF-301) or impermeable (ZIF-95X) for gases with a kinetic diameter bigger than or equal to the kinetic diameter of CO₂. Additionally, MMMs containing impermeable graphene nanosheets were fabricated as reference to confirm the ZIF-95X impermeability. The MMMs containing the ZIF-301 nanoplatelets showed enhanced CO₂ permeabilities. Analysis of the N₂ and CO₂ solubility and diffusivity showed that this permeable additive enhances the solubility of both gases, but only increases the N₂ diffusivity through the ZIF-301 MMM. This signified that for MMMs with sheet-like MOFs the MOF micropore volume should only be accessible for one gas such that the sheet-like morphology effectively increases the tortuosity for the other species. Contrarily, the MMMs containing impermeable ZIF-95X and graphene showed declined MMM separation performances. Analysis of the N₂ and CO₂ diffusivities showed the presence of defective interfaces in these MMMs and proved that an increased tortuosity is only effective if the additive-matrix interface is defect free.