The degradation of three aromatic thermoplastics copolymers of polyethersulfone (PES) and polyetherethersulfone (PEES) with the same composition but different sequence distribution was performed in a thermogravimetric analyzer in both nitrogen and air environments, in isothermal and dynamic heating conditions. The obtained results suggest that the degradations started by random chain scission under all the experimental conditions. Under N-2 flow, branching and crosslinking were Superimposed on the initial process, whereas complete oxidative degradation occurred in air. The apparent activation energies associated with the first degradation stage were evaluated and correlated with the sequence linkages present in the copolymer chains. The obtained values indicated that the chemical reactions occurring under nitrogen were different from those in air. In addition, a comparison of activation energies of PES/PEES copolymers with different sequence distribution was also reported. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 833-838, 2009