The thermal radiators are a major subsystem of the Space Station Freedom (SSF) Active Thermal Control System (ATCS). They dissipate to deep space the excess heat transported from the modules and truss mounted equipment. Condensation of the ATCS twophase working fluid occurs directly in small diameter tubes which are bonded to a thin aluminum face sheet in the flow-though radiator panels. The Permanently Manned Capability (PMC) configuration of the Space Station will have a total of 48 radiator panels grouped in 3 replaceable units of 8 panels on each side of the Space Station.Accurate prediction of radiator performance on orbit is important to keep the ATCS from getting too hot (exceeding its capacity) or getting too cold (freezing). For this reason, detailed models of the radiator system are being developed using the SINDA/FLUINT thermal and fluid systems analyzer. The models are currently being used to help assess the system design and to determine what design margins are required in order to account for uncertainties. SINDA/FLUINT models of the entire ATCS will continue to be used throughout the operations phase of Space Station Freedom to predict nominal and off-nominal ATCS performance.The SINDA/FLUINT model developed in this study has 70 fluid lumps and 340 thermal nodes. There is a simplified representation of the heat acquisition portion of the ATCS with significant detail in the radiator panels. One radiator fluid passage of each low temperature ATCS loop is modeled since the flow tubes alternate between loops A and B. There are 10 fluid lumps along the length of the flow tube in order to use the correct local fluid properties in the heat transfer correlations and to determine the point of complete condensation. There are a total of 90 face sheet nodes for the adjacent flow tubes modeled. This allows for dynamic calculation of fin efficiency under different conditions. Output from the model includes radiator panel temperature distribution in three dimensions, condensate flow rate and outlet temperature, location of complete condensation for each loop, local and overall face sheet fin efficiency and total radiator heat rejection.Many factors affect the performance of the ATCS, but detailed thermal/fluid modeling of the radiators leads to a reduction in the design margin due to uncertainty. Some questions which have been answered include the effects of fluid flow distribution due to plumbing geometry, uneven thermal environments, uneven thermal loop setpoint temperatures and uneven heat load distribution between low temperature loops A and B which flow through the same radiator. Of these, uneven heat load distribution between the two low temperature loops was found to contribute the most to reduced ATCS capacity.