We have developed a sensitive interferometric technique for measuring the temperature derivative of the refractive index of liquids with an accuracy of 0.5% as illustrated by measurements with mixtures of liquid n-hexane and toluene. In addition the concentration dependence of the refractive index of these mixtures has been measured with refractometry. The experimentally observed temperature dependence of the refractive index appears to be consistent with a modification of the Lorentz-Lorenz relation proposed by Looyenga.