It is proposed that molecular‐ion impurities that substitute for the halide ion in alkali‐halide crystals constitute an important class of impurities limiting infrared transmission in laser‐window materials. A survey of the experimental literature indicates that concentrations of less than 0.1 ppm ofNO2−, HCO3−, SO42−, andCrO42−will yield an absorption coefficient &bgr;>10−4cm−1at 10.6 &mgr;m. Predicted temperature dependences of impurity‐limited &bgr;(10.6 &mgr;m) areT1.6, nearly temperature independent, and decreasing &bgr; with increasing temperature depending on the particular ionic impurity.