A round robin interlaboratory comparison of thermal conductivity measurements was performed using the Guarded Hot Plate apparatus at temperatures from about 0°C to 1000 °C. There were twelve participating laboratories including four national laboratories in three different countries. A statistical analysis was performed and the variation in measurement results is discussed. Both within laboratory and between laboratory variability is analyzed. A definite conclusion of this comparison is that measurement variation increases progressively with increasing temperature. As a result there is a need for the U.S. national laboratory, the National Institute of Standards & Technology, to develop high temperature testing capability and then high temperature thermal conductivity reference standards.