An instrument for measuring the change in refractive index with temperature of solid infrared transmitting materials over the wavelength range 8 mu m to 13.5 mu m is described. A Littrow spectrometer arrangement is used with the prism sample mounted in an evacuated chamber with temperature control between -50 degrees C and 200 degrees C. The angular position of the spectrometer collimating mirror is servo-controlled to maintain the alignment of the outgoing and returning infrared beams from the prism. Measurement of the angular movement of this mirror then allows changes in the refractive index of the prism with temperature to be determined. The performance of the instrument is discussed in detail and results of measurements on germanium given; uncertainties in the values of the thermal coefficient of refractive index are considered unlikely to exceed +or-1.2%.