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The heat resistance of ethylene propylene co- and terpolymer vulcanizates has been studied over temperatures from 212F (100C) to 347F (175C) and exposure times up to and sometimes exceeding 1000 hours. Most established heat aging analyses rely on separate treatment of two parameters, retention of tensile strength or retention of elongation, to predict acceptable performance. This paper discusses a method with which to study aging behavior that combines these parameters into a single expression, the fractional strain energy at break, which describes an aged vulcanizate in terms of both its integrity (retained tensile) and its extensibility (retained elongation). Use of fractional strain energy at break is shown to be a reliable means of characterizing and projecting ethylene propylene aging behavior. Compounding principles which affect ethylene propylene long term heat resistance are demonstrated in the paper.