The gas permeability and ideal selectivity in poly(oxyindole biphenylylene) polymers that bear a tert-butyl carbonate side group, PN-BOC, have been measured, at 35 °C and 2 bar. It is found that the thermal degradation of the thermo-labile side group, BOC, at moderate temperatures and times, is useful for designing membranes that overcome the typical trade-off between permeability and selectivity. TGA, FTIR-ATR, and DSC analysis reveal that thermal treatments of PN-BOC at 150 °C and short periods of time, 5-60 min, allow the design of membranes with better selectivity-permeability combinations than the pure PN-BOC precursor. For the gas pairs O2/N2 and CO2/N2, the PN-BOC5 thermally treated membranes for 5 min show O2 and CO2 permeability coefficients higher by a factor of 2.3 with respect to that measured in PN-BOC, while retaining their selectivity. For the gas pairs H2/CH4 and CO2/CH4, the PN-BOC60 thermallytreated membranes for 60 min show a H2 and CO2 permeability coefficient higher by a factor of 1.7 and 1.4, respectively, while selectivity increases with respect to that of the PN-BOC precursor by 45% for H2/CH4 and 24% for CO2/CH4.