The thermal degradation of poly(arylene sulfide sulfone)/N-methylpyrrolidone (PASS/NMP) crystal solvate was studied by thermogravimetric analysis (TGA) and was compared with pure PASS in order to determine the way in which the formation of the crystal solvate affected the thermal properties of the polymer. The activation energy of the solid state process was determined using Kissinger’s method, which does not require knowledge of the reaction mechanism (RM), to be 174.18 kJ/mol which was lower than that for pure PASS (E = 214 kJ/mol). The study of master curves together with interpretation of integral methods, allows confirmation that the thermal degradation mechanism for PASS in the crystal solvate system is a decelerated Rn type, which is a solid-state process based on a phase boundary controlled reaction, in the conversion range considered. Whereas, the pure PASS follows a decelerated Dn thermodegradation mechanism in the same conversion range.