[en] It is stated that spectral features at 3.07, 10, 20 and 33 μm have proved valuable in the chemical characterisation of interstellar dust, and it is thought that as observations are extended into the far infra red other useful features will be observed. This note suggests that relatively sharp spectral lines may arise from neutral or ionised atomic species in interstellar dust at mm and cm wavelengths, and the detection of such spectral features would increase information on the chemical composition of interstellar dust. The 21cm line of the atomic H is discussed, and it is stated that H atoms can be found in many types of solid surfaces when these are subjected to nuclear irradiation. A list is given of hyperfine transitions of H atoms in some solids of atmospherical interest. H atoms could be formed in interstellar grains through the action of cosmic rays or photons from the interstellar UV radiation field. Alternatively, H atoms from ambient gas may collide with and stick to grains. Data are also given on transitions of Fe³⁺ and Cr²⁺ ions in various crystals. It is thought that transitions of Fe³⁺ and other transition metal ions in interstellar dust may be observable as absorption features in typical interstellar dust clouds. Although emphasis is on the possibility of detecting absorption features attributable to dust in the mm. and cm. regions it is thought that it may be more practical to search for emission features. (U.K.)