Hot-melt polyphenylethersulfone (PES) adhesive was used to link ethylene-propylenediene terpolymer (EPDM) elastomer, which can protect geothermal drill pipes, to a stainless steel (SS) substrate.These joints then were exposed for up to 70 days in steam at 200 °C to evaluate the durability of the bond. Although the preparation and modification of EPDM and SS surfaces played an important role in ensuring a strong bond in the EPDM-to-SS adhesive joint system, the susceptibility of the PES adhesive to hydrothermal oxidation led to conformational transformation of its sulfone group into a fragmental sulfonic acid derivative during the exposure.Moreover, a prolonged exposure time caused the decomposition of polyphenylethersulfonic acid derivative, forming two additional derivatives, aryl radical and H2SO4. Hot H2SO4 favorably reacted with Fe in the SS to yield a water-soluble Fe2(SO4)3 reaction product. This reaction product generated at the interfaces between PES and SS caused a decrease in peel strength. In fact, the loss of adhesion occurred in the SS adjacent to the PES.