Fused silica is the second of a series of materials to be certified as thermal expansion standards by the National Bureau of Standards. The results of tests on five samples indicate the stock to be consistent quality suitable for certification as Standard Reference Material 739. The same interferometer apparatus was used as in the certification of copper SRM 736. Expansivity values from each of the five samples were fitted using a least squares third order spline polynomial with three separate temperature intervals. The data from all the samples were then pooled and fit with the same type spline polynomial. Values calculated from these equations and their integrals are presented and also compared to literature data on fused silica.