The incorporation of small amounts of 3,5-diaminobenzoic acid (DABA) comonomer (usually 5-20 mol %) into an ortho-hydroxypolyimide (HPI) precursor constitutes a new methodology to easily improve the gas transport performance of thermally rearranged (TR) polymer membranes. Thermogravimetric analysis (TGA) profiles of HPI with DABA copolyimides (HPID) suggests that there is an overlap between the temperature ranges of two processes: the degradation of DABA carboxyl groups and the actual thermal rearrangement process. During thermal treatment at 450 degrees C, carboxyl pendant groups in DABA degrade while a more rigid biphenyl cross-linked structure is formed either following or at the same time as the rearrangement of imide to benzoxazole. Mixed-gas CO2/CH4 (1:1) experiments proved this new series of cross-linked TR poly(benzoxazole-co-imide) (XTR-PBOI) membranes as superior materials for CO2/CH4 separation applications, with enhanced permeability and selectivity, as well as high resistance to plasticization up to 40 bar.