AbstractThe infrared transmission spectra of NaI and KI have been recorded on a Beckman IR‐11 spectrophotometer using nujol mull technique at room temperature. Besides the fundamental absorption bands at the centre of the Brillouin zone, other lattice bands involving the interaction of infrared radiation with more than one phonon in the range 33 to 300 cm−1 have been observed. These multi‐phonon combinational bands (sum or difference) are explained in terms of the four characteristic phonons at or near the zone boundary and are consistent with the selection rules.