The effects of selected alloying elements on the elevated temperature tensile properties of tungsten fiber reinforced composites were studied. Composites were made of copper, copper- 2 percent chromium and copper- 10 percent nickel reinforced with various volume percents of uniaxially oriented tungsten fibers. The composites were tested in tension at temperatures up to 1800 degrees F.A comparison of the elevated-temperature tensile properties of tungsten fiber - copper alloy composites, representing soluble systems, was made with tungsten fiber - copper composites, representing a mutually insoluble system. The effects of alloying on the elevated-temperature tensile properties of tungsten fiber reinforced composites was also studied. A linear relation existed at elevated temperatures between tensile strength and volume percent fiber content for all the systems investigated. The copper alloy composites were weaker in tension at high volume percent fiber contents than copper composites in which alloying with the fiber did not occur. The tensile strength of the alloyed tungsten fiber decreased with increasing penetration of the alloying element into the tungsten fiber.