The index of refraction of Cd0.9Mn0.1Te was measured at a wavelength of 10.25 &mgr;m at room temperature on twinned unoriented monocrystal samples. The result wasn=2.57±0.01. Furthermore, the temperature dependence of the index of refraction was measured to be (1/n)dn/dt=5.0±0.1×10−5K−1. The optical absorption coefficient was &agr;=0.13 cm−1. Finally, the linear expansion coefficient in the range of 293–373 K was found to be &ggr;=−1±1×10−6K−1. ©1995 American Institute of Physics.