SCIENTIFIC ABSTRACT The ISO/LWS opens a unique window on molecular, atomic and ionic lines (as well as dust bands) in molecular cloud/HII region complexes, in a spectral range which is still essentially unexplored. The wavelength coverage of the LWS (45 - 196 um) enables to measure in the same observing conditions transitions of various species associated with the different phases present in the dense parts of the interstellar medium (molecular, atomic, ionic) making the study of relative intensities possible and, hence, the study of the chemical and physical conditions in a coherent way. The most important cooling lines of the warm molecular gas ([CII], [OI]) will be accessible together with transitions of O and C bearing molecules such as OH, the high transitions of CO and for the first time through some of the fundamental transitions of water. The main goal of this proposal is to improve our knowledge on both the physical and chemical conditions pertaining in galactic molecular cloud/HII region complexes. It is proposed to use the ISO/LWS to study the dense - n>10(3)cm-3 - parts of the interstellar gas. When present, the ultraviolet radiation of nearby (young) stars permeates the neutral gas with photons less energetical than 13.6 eV leading to photodissociation and heating of the molecular/atomic gas. The cooling of this gas is then dominated by a variety of strong atomic and molecular far-infrared lines. The frequent presence of shocks is another heating source for the gas with subsequent cooling via far-infrared lines. OBSERVATION SUMMARY In order to study in detail the physics and chemistry of this dense, warm neutral gas, we selected a number of prominent nearby molecular clouds in our Galaxy and the Magellanic Clouds spanning the range from high to low mass star formation, one quiescent molecular cloud, one Supernova Remnant and a few reflection nebulae. Full LWS grating spectra will be acquired for these regions along carefully selected directions probing the various physical/chemical conditions of the atomic/molecular phases. In addition, we selected strong point-like infrared sources where we intend to carry out high spectral resolution observations of some atomic and molecular lines both in emission (compact HII regions) and in absorption (embedded young stellar objects). These observations are proposed to analyze in greater detail the low lying transitions of H2O and other important molecules only accessible in the LWS wavelength range (O2, HD, HDO, OD) and address some fundamental problems related to the influence of on-going star formation on the neighbouring neutral gas. Systematically a full LWS grating spectrum at each position so that, if present, the broad features due to dust particles will be observed. The results of the present observations together with some other LWS proposals (evolved stars and PNe) could be combined to create a ISO LWS database on dust properties. Such a data base, even for a small number of typical sources, is a prerequisite for a further understanding of the chemical composition of interstellar dust.