The thermal degradation of saturated chlorinated polyethylene in powder and fiber form has been studied. Direct observations of fiber length variations have enabled us to prove the reactional mechanisms proposed. In the temperature range of 200°–300°C, the material exhibits some fluidity. Its degradation is explained by intrachain and interchain dehydrochlorination reactions, followed by an arrangement of conjugated double bonds obtained in the shape of polyacenic cycles. Between 300°C and 800°C, the polyacenic cycles react with each other to give a pregraphitic structure which is responsible for the increase in rigidity of the material. The fibers of saturated chlorinated polyethylene can be used as a precursor of carbon fiber.