Crystals of highly soluble metal-organic polyhedra 18 (MOP-18) were synthesized and used as additives in Matrimid ® 5218 to form mixed-matrix membranes. The MOP-18 linker was functionalized with long alkyl chains to help the crystals dissolve in common organic solvents. Scanning electron microscopy images revealed that the MOP-18 molecules do not aggregate into large particles even at loadings as high as 80wt%, and the absence of polymer veins or polymer rigidification at the MOP-18/polymer interface was attributed to the improved dispersion of the MOP-18 molecules in the polymer matrix. The increase in the modulus of the membranes from 1.4GPa for the pure polymer to 2.4GPa upon incorporation of MOP-18 was attributed to the strong affinity of the polymer chains for the alkyl chains of MOP-18. Gas permeation results showed that the membranes became more permeable as the temperature was increased from 35 to 70°C and that CO 2 plasticization, which occurred at 35°C and 8bar, was minimized when the membranes were heated to 70°C owing to an increase in the plasticization pressure to 21bar. At 70°C, the H 2 /CO 2 selectivity remained constant when the transmembrane pressure was increased from 3 to 30bar. The permeability and solubility data at high pressure and high temperature showed that the pore, core, and alkyl chains of MOP-18 introduced new sorption sites that significantly affected the gas transport properties of the membranes. The results also demonstrated that the MOP-18/polymer interface was mechanically stable at high pressure.