SCIENTIFIC ABSTRACT We propose to use the ISO-SWS to observe a number of lines of gas-phase interstellar and circumstellar molecules in the 2.5-45 micron range. The objective is to study the abundance and excitation of these species in a variety of sources and to assess their diagnostic capabilities both for the chemical and physical conditions. Key molecules for observation are H2, HD, H2O, OH, CO2, C2H2, HCCCCH and CH3. We propose to carry out a significant fraction of these observations at the beginning of the ISO mission so that the results can be fed back into the other SWS observations. The total amount of time for this proposal is 29.75 hours in case of the Sagittarius hole (spring launch) and 29.25 hours in case of the Orion hole (autumn launch). The proposal is divided into three parts: 1. Absorption line studies of vibration-rotation lines of gas-phase interstellar molecules. Most of the observing time will be spent on this part of the proposal, and it is the gas-phase complement of the Whittet et al. dust proposal. The emphasis will be on molecules which have bands in the wavelength region covered by the Fabry-Perot. The most important species are CO2, which is expected to be a sensitive diagnostic of grain-surface chemistry; CH3, which is an important cornerstone of the carbon chemistry network; and C2H2 (acetylene) and HCCCCH (di-acetylene), which are both good chemical and temperature diagnostics. These molecules (and their isotopes) cannot be observed through millimeter emission lines, and most of their infrared lines lie in wavelength regions that are not observable from the ground. In addition, for most lines of sight observations of the principal rotational lines of H2 and in some cases HD will be attempted. Molecules such as C2H, HCN, SO2 and SiS have already been detected through millimeter observations, but the infrared absorption lines will give valuable complementary information on the excitation. A few more complex organic molecules such as ethane will be searched for in only 1-2 sources. The H2, HD, CO2 and H2O observations toward Orion IRc2 or SgrA-W will be performed in collaboration with the Garching group. In order to constrain the elemental abundances, we have added observations of the fine-structure lines of O I, C II, Fe II, Fe I, Si II and S I. 2. Absorption lines of H2, CO2, CH3, C2H2 and HCCCCH with the Fabry-Perot toward IRC +10216. 3. Emission line studies of H2, HD, OH and H2O pure rotational lines and atomic fine structure lines in selected objects. The amount of time for this part will be about 4-5 hours. The emphasis is here on the diagnostic capabilities of these molecules in a variety of objects (including shocks, PDRs, and star-forming regions), for comparison with excitation models already developed by the authors. OBSERVATION SUMMARY The lines and bands of the various molecules that we propose to observe are summarized in the Tables at the end of the Scientific Justification section. For the absorption and circumstellar part, the integration times are such that a S/N of at least 100 (but often significantly more) on the continuum is reached with the grating. For the Fabry-Perot, the required S/N on the continuum is typically 50-100, and occasionally up to 200 (H2 and CO2 toward W3IRS5, IRC+10216). The grating observations will be observed in parallel with the Fabry-Perot observations using SWS AOT07. Complementary observations of the OH, H2O, O and HD lines at longer wavelengths will be made with the Fabry-Perot LWS AOT04. The integration time per LWS line is taken to be 2 min, so that a S/N>100 on the continuum is reached for all sources (using 3 res elements on either side of the line with 4 spectral samples per resolution element in the fast scanning mode). For the emission part, the requested S/N varies from 10 (H2 lines at 2 mu) to >100 (H2 lines at 17, 28 mu). A few of the strongest lines will also be observed with the Fabry Perot. At longer wavelengths, a complete medium resolution line scan with the LWS AOT01 will be taken for IC 63, IC 443 and DR21 FIR1. In addition, for IC 443 and DR21, LWS Fabry-Perot measurements of lines of OH, H2O, HD and O will made with AOT04, whereas OH, HD and O lines will be observed for HH 7. The integration time per LWS line is again taken to be 2 min, resulting in S/N>10 on the expected line strength. Note that the predicted strengths of the H2O and OH lines are particularly uncertain, due to unknown abundances, excitation effects and beam filling factors. Concatenation is required for all SWS AOT02 and AOT07 observations to ensure that the orientation/position angle of the SWS slit on the source is the same. It is recommended for SWS and LWS observations on the same source to save overhead for slewing. Please contact the PI in case of scheduling problems due to long integration times.