Thermal rearrangement of polyimides with OH side groups results in insoluble products (TR polymers) that reveal extremely high gas permeability combined with good permselectivity. However, the mechanism of the formation of these polymers remains unclear, as well as there are some doubts about their chemical structure. In order to elucidate this problem, infrared (IR) spectra of TR polymers were recorded and compared with those of several polyimide precursors. IR spectra in the range 400-4000 cm(-1) showed numerous bands that evidenced the formation of aromatic lactam structure in the final products of thermal rearrangement, while only weak bands indicated the presence of benzoxazole structure. It was also shown that thermal rearrangement of functionalized polyimides studied leads to highly permeable TR polymers: permeability coefficients (P) increased by a factor of 5-20, compared to those of the polyimide precursors.