When the temperature of an optical element changes, both its physical dimensions as well as its index of refraction change as a result. When designing an optical system that operates across a broad range of temperatures, knowledge of these thermal properties is critical. An interferometric method is presented for simultaneously measuring both the coefficient of thermal expansion (CTE) and the temperaturedependent refractive index (dn/dT) of materials in air. Any instrument that characterizes temperature-dependent properties must itself be largely insensitive to those same temperature changes. The design and realization of such an interferometer is detailed, highlighting the methods used for mitigating vibrations and undesirable temperature gradients. A discussion of measurement results on a Mitutoyo steel gauge block and CaF2 is also included.