This work presents the results of polymer membranes whose permeability and selectivity combination of properties can be defined through the chemical modification, in the solid state, of the repeating unit of a poly(oxindolebiphenylyene) that bears a -CH2OH group, PNCH2OH. The chemical modifications are promoted by moderate thermal treatments (160180 degrees C) and short times (<3.5 h). With TGA and FTIR-ATR, it is demonstrated that at certain levels of carefully controlled temperatures and times, the repeating unit of PNCH2OH is being probably transformed into random copolymers [(PNCH2OH)m(PNH)p]n and eventually at finite time into a new poly(oxindolebiphenylylene) in which the -CH2OH group is replaced by -H to produce a PNH polymer. The gas permeabilityselectivity combination for the gas pairs H-2/CH4 and O-2/N-2, at 35 degrees C and 2 bar, assessed for the thermally rearranged membranes falls slightly above or on the well-known additive behavior expected for copolymers depending on temperature and time and of course on type of gases to be separated. At 160 degrees C and short reaction times (5 min), the thermally modified membranes possess both higher permeability and selectivity for the gas pair H-2/CH4 than the nonthermally treated PNCH2OH membranes. At 180 degrees C and 30 min, the thermally treated membranes possess higher permeability coefficients for H-2 and O-2, by a factor of 3.3 and 4.0, and still have the selectivity of pure synthesized PNCH2OH.