Metal–organic framework 5 (MOF-5) nanocrystals with a high surface area (3000m 2 /g) and high thermal stability (up to 400°C) were synthesized and added to Matrimid ® to form mixed-matrix membranes for gas separations. Scanning electron microscopy (SEM) images of the membrane cross-sections revealed significant plastic deformation of the polymer matrix owing to the strong affinity between the MOF-5 and Matrimid ® . At 30% MOF-5 loading, the permeabilities of the gases tested increased 120% while the ideal selectivities remained constant compared to Matrimid ® . Residual gas analysis of permeates of gas blends with different mixture ratios revealed an increase in selectivity for CH 4 .