A study of the thermal stability of the outstanding sorption properties of nanoporous-crystalline and amorphous phases of poly(2,6-dimethyl-1,4-phenylene) oxide (PPO) is presented. In particular, the structural changes, as induced by thermal treatments in the temperature range 100-270 degrees C, of two limit nanoporous-crystalline modifications have been investigated by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Although both PPO nanoporous-crystalline modifications are metastable, their crystallinity reduction is small for thermal treatments up to 200 degrees C. For the entire thermal treatment range, the uptake of benzene from vapor phase for the PPO nanoporous-crystalline samples is higher than for PPO nanoporous-amorphous samples. After thermal treatments above the polymer glass transition temperature (T-g approximate to 220 degrees C), the guest solubility becomes negligible for the fully amorphous samples, while remains large for the nanoporous-crystalline phases, up to the polymer melting (T-g approximate to 250 degrees C). This high thermal stability of sorbent materials based on PPO nanoporous-crystalline phases is particularly relevant for their thermal regeneration processes.