The effect of thermal treatment and nuclear irradiation on the far infrared spectrum of polycrystal CsI was studied using pure and cerium doped crystals. Samples were exposed to neutrons and gamma radiation from a neutron activation tube. Thermal treatment was conducted in a 5.5 kilowatt furnace. Ionizing radiation caused significant infrared transmission losses at doses less than or equal to 1 MRad and the threshold wavelength of transmittance shifted to longer wavelengths. Transmission losses in cerium doped crystals were less than in pure crystals. Heat treatment also caused a significant reduction in transmittance. These results indicate that high temperature annealing will not decrease radiation damage.