Improving the heat resistance of thermoplastic polyurethane (TPU) is still a challenging task. Therefore, a better understanding of the processes contributing to thermal aging of these materials is of crucial importance. Previous investigations showed that thermal oxidation of the soft segments plays a major role in chemical aging of TPU. With the help of several analytical methods, we could prove that there is also a contribution by thermal oxidation of the methylene diphenylene (MDI) units, producing benzophenone derivatives. The amount of rigid material diminishes as a consequence of the MDI oxidation. The size as well as the amount of the rigid phase was found to be only one third after annealing in air at 150 degrees C. We conclude that besides oxidation of the soft segments, oxidation of the MDI units plays a major role in chemical aging of TPU. (C) 2000 Elsevier Science Ltd. All rights reserved.