Elastomeric vulcanizates of chlorosulfonated polyethylene (CSM) exhibit wide variations in heat resistance. Differences also occur among polychloroprene (CR) vulcanizates. These differences in heat resistance are related to compounding variables. For both CR and CSM, a minimum of polymer dilution with fillers and heat stable plasticizers is necessary. Other critical variables include the proper selection of metal oxides and antidegradants. Using an absolute elongation at break of 100% as the index or end point of elastomeric utility, the best heat resistant CSM vulcanizate has approximately 28°C (50°F) greater thermal stability than the best heat resistant CR vulcanizate. Heat resistant vulcanizates of CSM vary in thermal stability by 20-22°C (36-40°F), depending on their makeup. When air oven aged at constant temperature, differences of 3- to 5-fold in the time to reach 100% elongation are shown for CSM and for CR vulcanizates.