By employing high resolution H-1 and C-13 NMR spectroscopy combined with elemental analysis and FTIR-ATR, we have determined the basic chemical structure of Extem XH 1015, a new brand of polyetherimide with good thermal, mechanical properties, and processability. Bisphenol-A dianhydride (BPADA) and diamino diphenyl sulfone (DDS) are found to be the monomers for this newly developed polyetherimide. The gas permeability of this new polymer is reported for the first time in the literature. Polysulfone (PSU) and Ultem are employed as reference samples for the elucidation of permeability and selectivity differences among them because of their structural similarities. In addition to qualitative comparison of chain rigidity and packing with gas transport properties, computational simulations powered by Material Studio are performed at a molecular level to quantitatively investigate the relationship between the fractional accessible volume (FAV) and gas permeability. The FAV differences among these polymers increase with an increase in gas molecules diameters; thus these polymers have similar permeability for small gas molecules but diverse for large gas molecules. Their selectivity differences are also discussed in terms of FAV ratio. The FAV concept is proved to be more effective than fractional free volume to analyze and predict gas separation performance.