A critical analysis comparing the diffusion selectivity, solubility selectivity, diffusivity, and solubility coefficients of thermally rearranged (TR) polymers and polymers of intrinsic microporosity (PIM), two types of polymers which consistently perform at or beyond the polymer upper bound for certain gas pairs (O-2/N-2 and CO2/CH4), is reported here. Although most polymers in these two classes exhibit transport properties in the range of typical glassy polymers, several variants offer exceptional performance. The diffusivity selectivity for O-2/N-2 and CO2/CH4 for TR polymers and PIM lies outside the range of the glassy polymer database. The solubility coefficients for O-2 are at the upper end of the range of both TR and PIM polymers, as is also the case for CO2 solubility coefficients for PIM polymers. The O-2/N-2 and CO2/CH4 solubility selectivities for both PIM and TR polymers are in the range of typical glassy polymers. Thus, unique separation values for these polymers are a manifestation of maximizing diffusivity selectivity combined with very high gas solubility. Additionally, the previously determined diffusion gas kinetic diameters, dg, were found to correlate best with diffusion coefficients of PIM and TR polymers and serve to better analyze transport properties of upper bound materials.