The Coronal Diagnostic Spectrometer is an instrument designed to take measurements of the solar corona and is therefore pointed directly at the sun. The structure must be maintained within tight temperature limits to prevent the loss of image quality due to thermal distortions. A charged couple device (CCD) detector must be cooled below -80°C to give adequate performance and spacecraft interface considerations have also imposed severe requirements on the thermal control subsystem. One particular aspect of the instrument design is the need for careful contamination control and this has been considered a high priority in material selection. All these features have made the design of the thermal control subsystem an interesting and challenging problem which is described in this paper together with some of the analysis results which supported the design.