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Mixed-matrix membranes (MMMs) were formed by incorporating M 2 (dobdc) (M = Mg, Ni; dobdc 4− = 2,5-dioxido-1,4-benzenedicarboxylate) metal−organic framework (MOF) nanoparticles in a series of poly(ether-imide) copolymers. Addition of the MOF nanoparticles improved the permeability of H 2 , N 2 , CH 4 , and CO 2 relative to the pure copolymer by increasing gas solubility and, in most cases, diffusivity. More limited improvements in diffusivity were observed for the more strongly adsorbing gases. Because of such transport considerations, improvements in permeability and selectivity were most pronounced for H 2 /CH 4 and H 2 /N 2 separations. Incorporation of a greater ether content within the copolymers led to the formation of defect-free MMMs by physically sealing polymer−MOF interfacial defects, allowing higher MOF loadings to be achieved. For Mg 2 (dobdc), selective, defect-free films could be formed with loadings of up to 51 wt %. However, at these high loadings, films became weak and brittle. The mechanical properties of the composite materials were therefore quantified by tensile tests and compared to those of the neat polymers used commercially for membrane film formation. High contents of flexible ether units and small MOF nanoparticle sizes were found to be necessary to form strong and ductile MMMs, although clear trade-offs exist between transport performance, MOF loading, and mechanical properties. These trade-offs are critically examined to evaluate the current limitations and potential benefits to forming M 2 (dobdc) MMMs using this rubber toughening approach.