The facile fabrication of mesoporous materials using commodity polymers and improvements in the thermal and mechanical stabilities of the mesopores remain challenging. Herein, we report thermally stable, mechanically robust mesoporous monoliths made of commercially available poly(ether sulfone) (PES). A PES solution dissolved in N,N-dimethylformamide spontaneously formed a physical gel upon cooling to moderate temperature and/or mixing with benzene due to partial crystallization of PES. Mesoporous monoliths obtained from this gel had a reticular structure composed of nanofibers with an average diameter of 18 nm in conjunction with a high specific surface area of 206 m(2)/g. The dense reticular structure showed a large total pore volume of 2.95 cm(3)/g, including a significant mesopore volume of 0.52 cm(3)/g. The nanofiber structure together with the high glass transition temperature of PES significantly enhanced the thermal stability of the mesopores, 80% of which were maintained even after aging at 150 degrees C for 600 h.