Raman measurements between 77 and 340 K are reported for Ag2HgI4 and Cu2HgI4. Additively far infrared reflection and transmission spectra between 30 to 180 cm−1 are measured. Symmetrybased vibrational band assignments are given for both the internal and external optic modes. The conductivity in the region 30 to 150 cm−1 is calculated and the behaviour is explained using the jump diffusion model. Copyright © 1984 WILEY‐VCH Verlag GmbH & Co. KGaA