The development of a new generation of radioisotope power systems is presently underway using a variety of innovative material selections. One of these materials is the cobalt based superalloy Haynes Alloy 25 (L605) (Haynes International, Inc., Kokomo, IN) which is being proposed as a structural member in the fabrication of Alkali Metal Thermal to Electrical Conversion (AMTEC) cells for space applications and other proposed heat source applications. Even though Haynes Alloy 25 has been previously used in several space power systems, the new power systems presently being developed will have some operational characteristics outside of the known design envelope. Therefore, the future application of these new power system technologies is dependent on determining many of the long term high-temperature physical and mechanical properties of the base-lined materials. The emphasis of this paper is on the determination of some of the high temperature mechanical properties (yield strength, ultimate tensile strength, and percent elongation) and the material compatibility characteristics of Haynes Alloy 25. Haynes Alloy 25 compatibility capsules were designed and tested to determine its high-temperature compatibility with various proposed AMTEC cell materials (nickel, rhodium, and molybdenum) being considered for future applications. The results of the studies and material recommendations will be discussed.