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The environmental requirements of the Selenide Isotope Generator have led to a thorough evaluation of all available high temperature thermal insulation materials and the development of some new ones. This paper reports primarly on the results of thermal conductivity and load bearing experiments performed on a series of fibrous and cellular insulation materials capable of operating at hot face temperatures in the 800 to 1000 C range. The thermal conductivities of fourteen materials were determined using the guarded hot plate technique (ASTM-C177-77). All of the materials were tested in a vacuum environment, most were tested in helium, and a few were checked in xenon. The load relaxation characteristics of the end plug materials were determined using two different techniques. The first involved cross plotting the results of a series of creep measurements. The second utilized a strain gage to provide a more direct determination of the load relaxation of the sample. The data presented here allow comparisons to be made among these materials in a fairly confident manner, since all of these normally difficult measurements were obtained from the same apparatus.