The determination of the thermal Gru¨neisen parameter of anisotropic reentry vehicle materials has been limited in part by the accuracy of experimentally determined values of the thermal coefficient of linear expansion. In many cases, the coefficient of thermal expansion of recently developed anisotropic materials is very small and precise determination with commercially available dilatometers is, at best, extremely difficult if at all possible. A description will be given of a completely automated dilatometer developed at the Air Force Weapons Laboratory. The instrument employs samples of the materials of interest as the mirror spacing for a confocal, spherical Fabry‐Perot cavity. Results are presented of measurements made on well‐characterized, low expansion materials demonstrating a less than 1% uncertainty in the coefficient of expansion.