The use of sulfur-based plastics has led to the accumulation of end-of-life sulfur-based plastic waste that should ideally be recycled. In this study, several technologies were applied to investigate the thermal-oxidative degradation property of sulfur-based plastics (polyphenylene sulfide (PPS), polyether sulfone (PES), and polysulfone (PSU)). Thermogravimetric analysis (TGA) revealed that these plastics undergo major decomposition at 450–650 °C under N2, whereas two-stage weight loss at 450–690 °C was observed in air. X-ray photoelectron spectroscopy (XPS) and in-situ radical monitoring by heated electron spin resonance (heated-ESR) spectroscopy revealed that the samples had altered their chemical structures, and that radicals are involved in samples treated at low temperature (≤ 400 °C). The thermo-oxidative products were analyzed by customized pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). PPS pyrolysis mainly produced sulfur-containing aromatic compounds, with H2O, CO2, and SO2 released during two-stage degradation in air, which suggests that pyrolysis and oxidation occur simultaneously during the first stage. PES pyrolysis generated various oxygen-containing products, whereas the oxidation of PES resulted in a great number of furans and dioxins at the expense of phenolics. Extensive evolution of SO2 was initially observed, irrespective of the degradation atmosphere. PSU pyrolysis produced various aromatics, phenolics, and esters, with SO2 formed as the major sulfur-containing compound, which was released at a lower temperature (∼50 °C) in air compared to N2, confirming that PSU is less thermally stable than PPS and PES in air, as observed by TGA. The thermo-oxidative degradation behavior of sulfur-based plastics was comprehensively characterized by combining conventional techniques (TGA and XPS) with advanced analytical technologies (heated-ESR and customized Py-GC/MS).