A program is described for measurement of dimensional stability of fused silica and several ultralow thermal expansion materials. Two techniques are being used: Fizeau interferometry by NBS‐Corning and Fabry‐Perot interferometry by the Optical Sciences Center, University of Arizona. In the latter measurements an accuracy of one part in 109is sought. A by‐product of the Arizona results will be measurements of the stability of optical phase shifts on reflection from multilayer stacks and also dimensional stability of optical contacts.