Unique thermal management requirements imposed by the Space Station, such as high heat loads, long transport distances, and long operating life, have created the need for technology development in the area of heat rejection. Since 1979, the NASA Lyndon B. Johnson Space Center has been conducting a program to develop a modular heat pipe radiator subsystem for Space Station application. This paper discusses the thermal vacuum test program for two prototype radiator elements which was performed at the Johnson Space Center. Included is a basic description of the test articles, a development of detailed test objectives, a presentation of the test approach, and test results. It is shown that the fundamental design goals for the radiator subsystem have been met.