This Borosilicate Glass is the third Standard Reference Material (SRM) to be certified for thermal expansion measurements by the National Bureau of Standards. Copper (SRM 736) and Fused Silica (SRM 739) are the other two materials. The results of tests on six samples taken from the stock will be reported in this paper. These results indicate the stock to be of consistent quality suitable for certification. Measurements were made in the temperature range from 80 to 680 K using an interferometer apgaratus. At 293 K the expansivity is4.78×10−6 K−1and the standard deviation is0.06×10−6 K−1calculated from a least squares fit to the data over the entire temperature range. A description of the calibration of a fused quartz dilatometer using the three Standard Reference Materials will also be presented.