Rebar-couplers that mechanically splice pairs of steel reinforcing bars, end-to-end are used regularly in reinforced concrete construction. Epoxy-bonded couplers are one available type, but have unique long-term performance considerations. The adhesive material used in these couplers is a two-part, field-mixed, ambient-cure epoxy system, originally designed for adhesive anchorage to concrete. Many of the adhesive systems used for anchorage to concrete, including the system used with adhesive-bonded couplers, are epoxy systems. The mechanical properties of these types of epoxies have been shown to degrade over time, in the presence of moisture. A variety of commercially available adhesive systems, for anchorage to concrete, were studied to assess their relative resistance to moisture-based degradation. The material properties of two of the adhesive systems, both epoxies, and the performance of the rebar-couplers were then measured over a fourteen-and-a-half-month period of exposure to a variety of environmental conditions, including water immersion at a range of temperatures. From these results, material degradation models were used to predict the properties of the adhesive over the service life of the rebar-coupler. A Finite Element Analysis (FEA) model was developed to simulate the tensile failure of the epoxy-bonded rebar-coupler system and correlate degrading adhesive material properties to changes in the coupler system's behavior throughout its service life.