XC Associates proposes to build on prior work to develop and characterize a very high efficiency, lightweight radiator constructed from high thermal conductivity carbon fibers. The radiator will be suitable for use on instruments carried by future spacecraft. The phase I effort will address the thermal performance and size limitations of the composite design. Since cryogenic applications are becoming more common the thermal characteristics of the radiator will be measured down to 45K. Modern spacecraft thermal control needs to be low mass and low cost. Composite thermal radiators will realize a greater than 60% weight saving compared to aluminum and the costs can be addressed by the use of standard technology that can be easily applied to new designs. The major innovation to be demonstrated by this project is the use of highly orientated fibers that completely eliminates the low through thickness thermal conductivity normally associated with traditional carbon fiber laminates. This highly oriented lay-up also allows the thickness of the radiator to be tapered down to less than 0.020" while retaining very high stiffness. Therefore, the radiator is much thinner than the equivalent aluminum radiator, which combined with lower density of composite construction, accounts for the significant weight reduction.