This paper describes the design and performance characteristics of the thermal control system for the Spacelab Astro-1 payload. Dubbed the Integrated Radiator System, or IRS, the assembly is a thermal radiation system designed to maintain the temperature of payload electronics packages within a specified range, regardless of payload or orbiter orientation in space. The radiator is passive in its ability to reject heat, but active when the lower temperature limit is reached. At that point, resistance heaters are energized to maintain the radiator surface temperature within acceptable range.The performance of the radiator system during its flight aboard the Space Shuttle is reported in this paper. An earlier paper presented the thermal analysis and test data predicting the performance of the radiator system before the flight. In this paper, data collected during the flight is reported and compared to the performance predicted by analyses and tests.The radiator consists of two aluminum planes, connected at right angles, which are covered with silverized Teflon tape. The radiator structure is attached to the cruciform-shaped platform which supports a cluster of ultraviolet telescopes. The two faces of the radiator surface are connected by a series of fixed conductance heat pipes attached in orthogonal directions to the front and back sides of the radiator surface. The heat pipes were installed to maintain the radiator surface at a uniform temperature. Heat loads are generated by electronics boxes mounted directly to the back side of the radiator.The original design concept for the radiator is presented in brief, including its performance specifications. The testing which was conducted to verify the system performance is also presented. The test history of the heat pipes is given, along with the methods of detecting the presence of noncondensable gases within the heat pipes. The flight performance of the IRS proved to be nominal for the orbit conditions flown.