A series of poly(acyl sulfides) were synthesized by polycondensation of acid dichlorides with sodium sulfide using tetrabutylammoniumhydrogen sulfate as a phase transfer catalyst. The thermal stability of these polymers was determined by thermogravimetric analysis. A unique phenomenon of exothermic degradation behavior was observed by differential thermal analyses, which is attributed to the energetics of the acylsulfide linkage during the thermal degradation. The mechanism of formation of degradation products was investigated by the pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) technique.