Ethylene-propylene rubber compounds have been utilized in automotive radiator and heater hoses for many years. This is due to the elastomer having superior resistance to ethylene glycol coolant fluids, outstanding ozone resistance, and excellent heat aging characteristics. However, recent automotive-design and engineering trends have created severe temperature environment under the hood. As a result, EPDM compounds are now evolving to more polymer-rich compositions with increased heat resistance, lower compression set, and non-bloom properties.Compounding principles are described to meet future heat aging requirements of coolant hose. Compositional and structural factors of the polymer are shown to have an important influence in maximizing the heat resistance. Peroxide crosslinking, preferred for heat and oxidative stability over conventional sulfur, has fundamental problems associated with its use; systems are described to maximize the rate of cure and development of physical properties. Long term heat exposure has been studied from 150°C through 200°C to assess serviceability. Present and future recommended coolant hose compounds are compared under these conditions.