The radiation-induced optical transmittance loss in polycrystal Csl was investigated in the 2.5- to 40-μm infrared region. Exposure of polycrystal Csl to ionizing radiation doses in the 0.3- to 1-Mrad range using a research reactor causes a significant degradation of the optical transmittance. Losses of up to 50% over the infrared transmiffance spectrum are reported; however, changes saturate at ⪅ 1 Mrad. The loss is particularly evident at shorter infrared wavelengths, shifting the threshold wavelength for transmittance to higher wavelengths. Results also show that cerium-doped polycrystal Csl exhibits improved radiation resistance.