Nano/microsized MIL_-101Cr was synthesized by microwave heating of emulsions for the use as a composite with Matrimid mixed-matrix membranes (MMM) to enhance the performance of a mixed-gas-separation. As an example, we chose CO₂/CH₄ separation. Although the incorporation of MIL_-101Cr in MMMs is well-known, the impact of nanosized MIL_-101Cr in MMMs is new and shows an improvement compared to microsized MIL_-101Cr under the same conditions and mixed-gas permeation. In order to reproducibly obtain nanoMIL_-101Cr microwave heating was supplemented by carrying out the reaction of chromium nitrate and 1,4-benzenedicarboxylic acid in heptane-in-water emulsions with the anionic surfactant sodium oleate as emulsifier. The use of this emulsion with the phase inversion temperature (PIT) method offered controlled nucleation and growth of nanoMIL_-101 particles to an average size of <100 nm within 70 min offering high apparent BET surface areas (2,900 m² g⁻¹) and yields of 45%. Concerning the CO₂/CH₄ separation, the best result was obtained with 24 wt.% of nanoMIL_-101Cr@Matrimid, leading to 32 Barrer in CO₂ permeability compared to six Barrer for the neat Matrimid polymer membrane and 21 Barrer for the maximum possible 20 wt.% of microMIL_-101Cr@Matrimid. The nanosized filler allowed reaching a higher loading where the permeability significantly increased above the predictions from Maxwell and free-fractional-volume modeling. These improvements for MMMs based on nanosized MIL_-101Cr are promising for other gas separations.