SCIENTIFIC ABSTRACT In order to determine the composition of the dust in various types of clouds, e.g. molecular and diffuse clouds, we propose to measure the absorption spectra of large samples of (a) sources embedded in clouds, (b) stars located in the background of clouds, and (c) highly reddened field stars. Emphasis is placed on the study of ices (e.g. that of H2O, CO2, CO) and silicates. The optical depths of the absorption features of the various ices compared to that of the silicates allow to derive the chemical and physical conditions (e.g. temperature) within the clouds and to study the coagulation and accretion of dust grains. Attempts will be made to determine the intrinsic spectra of the embedded sources. OBSERVATION SUMMARY We want to study the chemical composition of dust in various clouds. For this purpose we selected a set of embedded sources from the literature (e.g., the IRAS Point Source Catalogue) and added a number of highly reddened stars to probe lines of sight through the diffuse interstellar medium as well as through several dark clouds. The spectra of the proposed sources will be recorded with PHT-S (AOT P40). The integration time is determined by the requirement of a S/N of at least 20 with the exception of the strongest absorption bands in a very limited number of sources.