The thermal degradation of poly(phenylene sulfide sulfone) PPSS was studied by thermogravimetricanalysis to determine the reaction mechanism. Several tests were carried out at different heating rates. The influence of the heating rate in dynamic measurements (5-40Cmin) on kinetic parameters, such as activation energies, was also studied. The activation energy of the solidstate process was determined to be 214kJ/mol using Kissinger's method, which does not require knowledge of the reaction mechanism. The value of the activation energy obtained using Flynn-Wall-Ozawa method was in agreement with that using Kissinger's method. Different reaction mechanisms were used to compare with this value. Also the experimental data were compared to master curves. Analysis of the experimental results suggested that the actual reaction mechanism was a Dn deceleration type.