VTECTM PI-1388 and 20 wt% (Al) NH2-MIL-53/VTECTM PI-1388 mixed-matrix membranes (MMMs) with different thicknesses were tested for gas permeation of H-2 and CO2 from 5 to 30 bar and from 35 to 300 degrees C. 50/ 50 H-2/CO2 mixtures were also tested at 30 bar and 250 degrees C with stage cuts that ranged from 0.05 to 1. Gas permeation data show that the affinity of (Al) NH2-MIL-53 for VTECTM PI-1388 is strong enough to perform gas separations under these conditions. At 30 bar and at temperatures above 200 degrees C, the performance of the MMMs for H-2/CO2 separation improved significantly with increasing temperature. Specifically, the H-2 permeability of the MMM at 300 degrees C increased by 70% with respect to that of VTECTM PI-1388 (VTECTM PI-1388: H-2 = 85 Barrer, H-2/CO2 = 4.0; MMM: H-2 = 144 Barrer, H-2/CO2 = 5.8). Gas mixture separations using VTECTM PI-1388 and the MMM depended on the stage cut and reached a maximum H-2/CO2 separation of 7.2 for VTECTM PI-1388 and 7.5 for the MMM at a stage cut of 0.05.