Microstructure of poly(ether ketone sulfone) copolymers I-V, derived from varying mol proportions of dihydroxy diphenyl sulfone (DHDPS, A) and dihydroxybenzophenone (DHBP, C) with stochiometric amounts of difluorobenzophenone (DFBP, B) was studied by C-13 nuclear magnetic resonance spectroscopy. The results were interpreted in terms of the compositional triads BBB, BBA, ABA, BAB, and AAB because B and C moieties become indistinguishable in the copolymers. Feed ratios calculated from the triad intensities agree well with experimental values, validating the chemical shift assignments. The presence of AAB and BBA triads in polymer II (A : C = 1 : 0) indicates the occurrence of transetherification reaction during its synthesis. Thermal decomposition characteristics of the copolymers were studied by thermogravimetry. Activation energies for thermal degradation were calculated using Coats-Redfern＇s method assuming the order of the reaction is 1 and was found to vary from 281 to 193 kJ mol(-1). A good linear correlation was obtained between activation energy values and BBB tried intensities. These observations were rationalized by consideration of their decomposition mechanisms.