Recently a high modulus CFRP system was developed to enhance the load carrying capacity and serviceability of steel bridges and structures. However, the environmental durability of the system has not yet been demonstrated. This paper presents the findings of a research program that was conducted to evaluate the environmental durability of the bond of the proposed CFRP strengthening system to steel surfaces. The program consisted of testing 44 steel-CFRP double-lap shear specimens. The specimens were exposed to severe environmental conditions for different durations, up to 6 months. Different methods to enhance the bond durability were studied including pre-treating the steel surface with a silane coupling agent, inserting a glass fiber layer within the adhesive and a combination of both methods of protection. The research findings indicate that the use of a silane coupling agent significantly enhanced the bond durability. While the presence of the glass fibers helped to enhance the initial bond strength of the system, it did not improve the durability of the bond. The use of both techniques enhanced both the overall bond strength and the environmental durability of the strengthening system.