Abstract The method of isomorphic substitution is applied to, and gives a fairly detailed assignment of, the infra-red spectra of silicates and germanates of the olivine type. Two regions are easily distinguished in the spectrum: the first (1000–1450 cm−1) is characterized by a fairly uniform pattern, highly specific of the SiO4 (or GeO4) tetrahedra involved in the olivine structure; these bands are assigned to the v1, v3 and v4 modes of SiO4 or GeO4 tetrahedra. The second region (450–1280 cm−1) is more or less influenced by the nature of the cation X and may be assigned, in part to vibrations of XO6 octahedra, in part to the v2 mode of SiO4 tetrahedra; this last mode has been identified by the study of a complex solid solution (Mg, Ni, Co, Mn)2SiO4. The deformation of the tetrahedral sites is proved by the splitting of the v3 mode in dilute solid solutions of the type X2(Si, Ge)O4. A non-linear, but fairly regular relationship exists between the SiO4 frequencies and the ionic radius of the cation X. The influence of isomorphic substitution on band contour is discussed for solid solutions (X, Y)2SiO4 and X2(Si, Ge)O4.