Abstract Lead sulfide has been found to form stable glasses with GeS 2 in the presence of GeS. Far-infrared transmission spectra of the glassy compositions x = 0.46, 0.23 and 0.115 in the pseudoternary (PbS) x (GeS 2) 0.70− x (GeS) 0.30 and of the stoichiometric crystalline composition Pb 2GeS 4 have been observed for the first time. The results indicate a two optical mode behaviour of the compounds. The presence of the heavy lead atom in the GeS network seems to appreciably affect the bond bending modes of the GeS 4 molecular unit without significantly disturbing its bond stretching vibrations. The presence of S 3GeGeS 3 molecular units in the amorphous compositions and their absence in Pb 2GeS 4 are indicated.