The temperature dependence of the reflection intensities from copper was examined in order to determine the mean-square vibration amplitudes of the atoms in the metal. The reflections from a prealigned single crystal were examined in a Norelco diffractometer outfitted with a high-temperature chamber designed by Chiotti. The intensities of the (400) reflection for copper were measured at temperatures from 13 to 1000°C. Difficulty was experienced in obtaining reproducible intensity measurements, and it is suggested that this was due to changes in the mosaic structure of the single crystal during thermal cycling.