The thermomechanical behavior (less than 1 Hz) of a newly available, high temperature, aromatic polysulfone has been investigated by the technique of lorsional braid analysis (TBA). Speculations concerning the synthesis and structure are discussed in terms of NMR and infrared spectroscopy, elemental analysis, and a review of the general literature on polysulfones. In addition to the glass transition at 287°C, a secondary relaxation was detected in the glassy state at −100°C by a mechanical loss maximum and change in modulus. The stability of the polysulfone at elevated temperatures in air and nitrogen as monitored by TBA, DTA, and TGA is reported. The influence of heat to 500°C, high doses of ionizing radiation, and water on the transitions of the polymer is discussed.