Most of the physical and chemical properties discussed thus far are impacted when an elastomeric compound is exposed to high temperatures. Whether these temperatures are inherent to the application or the result of frictional buildup, they can be dangerous, especially if the elastomeric lip is exposed for a prolonged period of time. Affected properties can include hardness, tensile strength, modulus, elongation, compression set, and lip volume.

Unless specially formulated, elastomers will typically soften when first exposed to high temperatures. Extended heat exposure can cause irreversible changes in tensile strength and elongation, as well as alterations in the chemical makeup of the compound such that it progressively and permanently hardens. This hardening is the result of additional cross-linking, plasticizer evaporation, and/or oxidation. A hardened seal is less flexible and therefore less able to follow any eccentricities in the shaft. This progressive inability to maintain proper contact with the shaft can result in leakage past the lip.

Two ASTM test methods for gauging high temperature effects are most used in relation to shaft seals. Both are designed to gauge the amount of material degradation that results from exposure to a heated environment. The difference between these two tests is mainly the device used to maintain pressure and heat on the specimen. ASTM D 573 details testing in an air oven, and ASTM D 865 describes heat and air testing within a test tube enclosure.

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