Polyester cord strength loss in EPDM depends on three factors: the presence of moisture, the nature of the cure system, and the aging environment. Of these factors, moisture is most critical. Cord degradation is caused by hydrolysis and is catalyzed by amine or amine-like compounds formed from the curvatives. High temperature also accelerates the deterioration. Knowledge of the service environment is required to decide whether PET may be used to reinforce EPDM. Moisture has a very harmful effect on PET degradation, and a dry environment increases service life substantially. The most feasible method of controlling PET cord strength loss is via curative choice to minimize or avoid nitrogen-containing curatives. Use of desiccant to remove moisture gives no improvement under normal service conditions. “Amino Nitrogen Index” is shown to be useful in formulating EPDM cure systems which have least effect on PET. Systems with an ANI of less than 6 give satisfactory performance. Curatives and accelerators commonly used in EPDM can be ranked according to this concept to aid future compound development. The peroxide system is suggested for maximum heat and compression set resistance. Based on the laboratory performance level of commercial EPDM/cord systems, PET can be used in properly-compounded EPDM at temperatures to 125°C in combined glycol/air and to 150°C in hot air. The cord strength loss of rayon, nylon, and aramid is independent of cure system amino nitrogen content. When the proper cure system is used, the durability of PET is comparable to aramid in hot air. PET is equivalent in performance to rayon and nylon up to 125°C in glycol/air, but far below aramid. Aramid, nylon, and rayon appear suitable for continuous exposure to 150°C glycol. However, in practical applications of these cords other factors such as intermittent environmental changes, dimensional stability, etc. must be considered.