An interferometric technique is described that uses a Fabry-Perot type configuration to measure the coefficient of thermal expansion (CTE) and the thermo-optic coefficient (dn/dT) of transparent materials concurrently. A number of measurements of the CTE and dn/dT are made for both oxide and fluoride based glasses. These measurements show that the technique is accurate, reproducible, and is very effective in determining the temperature dependence of both the thermo-optic and thermal expansion coefficients.