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SCIENTIFIC ABSTRACT The nature and evolution of interstellar dust, and its significance as an active ingredient of the interstellar medium regulating both physical and chemical processes, are key problems in modern astrophysics. It is proposed to use the ISO Short Wavelength Spectrometer (SWS) to investigate a number of key problems relating to the composition, structure and evolution of interstellar dust grains. The proposal is designed to exploit the unique capability of the SWS for high-resolution spectroscopy in the 2.4 to 45 mu waveband. This spectral range contains virtually all of the vibrational resonances arising in solids of astrophysical interest, and includes regions of high telluric absorption which are as yet unexplored. The spectral resolving power of the SWS is ideally suited to the project, allowing investigation of fine structure in the solid state features which provide important diagnostic tests of composition and annealing. A primary objective of the proposal is to determine the abundances of carbon-bearing molecules in grain mantles, including CO2, CH_4, CH_3OH, H_2CO and the nitriles (XCN). A new determination of the ^12C/^13C ratio will be made using the strong resonance of CO2 near 4.3 mu, and we will also investigate the deuterium abundance in the organic component of the mantles. Results will be used to assess the importance of grain surface reactions for gas phase and solid state chemistry in the interstellar medium, and to study the composition and evolution of the mantles. At lower priority, the structure of the silicate component of the dust will be investigated by detailed study of the 9.7 and 18.5 mu profiles. The question of whether interstellar silicates are significantly hydrated will be addressed by means of precision observations in the 2.6 to 2.9 mu waveband. Finally, the region from 25 to 45 mu will be explored in a search for new features. OBSERVATION SUMMARY A total of 31 targets have been selected for this program (Table 1). All have known interstellar dust features (ice, silicates) studied with ground-based telescopes. The proposal uses primarily the SWS: only nominal amounts of time are requested on other ISO instruments. Two SWS AOTs will be used: SWS01 (full 2.4--45 mu grating scans at reduced resolution) and SWS06 (wavelength-selective scans at full grating resolution without F--P). For the SWS06 observations, wavelength ranges have been selected to optimize coverage of important interstellar features at high signal-to-noise, consistent with the available time. The important interstellar resonances fall mostly in the short-wavelength part of the SWS spectral range, but use will also be made of the serendipitous long-wavelength observations that are acquired simultaneously. Sources in which interest focuses mainly on the broad silicate features will be observed exclusively with SWS01, and some bright sources will be observed with both SWS01 (to give ``overview'') and SWS06 (to give profile details of selected features). A summary of the integration times required for each source and AOT is given in Table 2 (Spring launch) and Table 3 (Fall launch). These have been estimated using the SOTE program and fluxes from the NASA Catalog of Infrared Observations and the IRAS Point Source Catalog. The total integration time allocated to the proposal is 20 hours.