AbstractThe thermal stability of PVC resins, alone or in powder blends, and of their various processing products can be determined continuously with precision and reproductibility. This new method is based on the evaluation of the progressive changes resulting from the decomposition of long test samples subjected to incremental thermal exposure in a thermo‐isolated oven kept at a constant temperature. The test samples are then examined by scanning reflectometry under white—as well as UV—light in order to detect and measure the structural changes occurring in the polymer during its thermal degradation. The method may also be applied to the quality control in the extrusion or calendering plant, where a scanning fluoro‐reflectometer detects the decomposition threshold during pro