Physical aging in amorphous polymers causes a decrease in specific volume and thus in the gas transport properties of their membranes. In this work, the effect of simultaneous thermal decomposition of a thermolabile tert-butyl carbonate group, BOC, and cross-linking by a propargyl group (-CH2-C equivalent to CH) on the gas selectivity-permeability properties of the resulting membranes is studied to learn how membranes with mitigated variations in the gas permeability coefficients with aging time may be produced. The model copolymer is a poly(oxyindole biphenylylene) that bears BOC and propargyl groups, [(PN-BOC)(x)-(PN-Pr)(y)](n). Systematic studies on the structure/processing/property relationship assessed by TGA, DSC, and permeation measurement using pure gases reveal that a single thermal treatment for 1 h at 240 degrees C on a neat copolymer membrane, 12-20 mu m thickness, is enough to produce chemically robust membranes (insoluble in NMP and DMSO) and that are physically more resistant to aging since the permeability reduction rate approaches zero. The cross-linked membranes possess lower gas permeability coefficients with higher ideal selectivity with respect to the corresponding neat copolymer membranes, i.e., the P(H-2) decreases from 60 to 42 Barrers but H-2/CH4 selectivity increases by a factor of 2 (21 to 40), and in general the selectivity permeability properties for the gas pairs H-2/CH4, O-2/N-2, and CO2/CH4 do not present drastic variations with aging time at least from 72 to 2000 h.