The degradation of a new thermoplastic polyethersulfone containing more than one SO2 group in the repeating unit was carried out in both dynamic and isothermal heating conditions, under N-2 flow and in a static air atmosphere. The experiments indicated that the polymer degraded by initial random chain scission, followed by branching and crosslinking in an inert atmosphere and complete degradation in an oxidative environment. Apparent activation energy values associated with the first degradation stage were calculated and suggested that the first degradation stage was not affected by the experimental conditions. Results indicated for the investigated polymer a thermal stability higher than those of some previously investigated polyethersulfones containing only one SO2 group in their repeating units.