The effects of bromine substitution on the physical and gas transport properties were examined for five families of tetra-substituted glassy polymers: bisphenol A polycarbonates (PC), hexafluoro-polycarbonates (HFPC), hexafluorobisphenol-tertiary butyl isophthalates (HFBP-tBIA), fluorenebisphenol-t-butyl isophthalates (FBP-tBIA) and bisphenol A-t-butyl isophthalates (BPA-tBIA). Additionally, the thermal response of the substituted BPA-tBIA polymers was explored to elucidate the effects of bromine substitution on the activation energies of permeation and diffusion and on the heat of gas sorption. Compared to its methyl-substituted analog, each bromine-substituted polymer had a higher cohesive energy density, a higher glass transition temperature and a lower oxygen-specific fractional free volume. Bromine substitution significantly reduced O-2 and CO2 permeability and substantially increased both O-2/N-2 and CO2/CH4 selectivity. An increase in the activation energy of diffusion for N-2 compared to O-2, and for CH4 compared to CO2, may be credited for the majority of the selectivity grain. A new method for evaluating penetrant-dependent fractional free volume proved to be a valuable tool in determining the effects of structural changes on gas transport behavior.