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==> In this proposal, more time is requested for JCERNICH.MAS_AGUA For many years the extent of water emission and its abundance in molecular clouds have been key problems in the study of the interstellar medium (ISM). Although all chemical models predict a high water abundance, ground-based observations of non-masering rotational transitions of water are practically impossible. Recently, Cernicharo et al. (1990, 1994) succeeded in observing at 183 GHz the weakly masering 313-220 transition of water in many molecular clouds opening a new window to study water in the ISM. In particular, it was shown for the first time that water emission in Orion is spatially extended. The derived abundances demonstrate that water is one of the most abundant molecular species in warm molecular clouds. In the ISO proposal CERNI_1, for which we obtained high priority time, we have observed SgrB2 and obtained spectacular results concerning the water extent and its excitation conditions. In the recent observations of Orion we have detected with the LWS-FP 15 pure rotational lines of water vapour in emission (see Fig. 1 and 2). In addition, our FP and grating rasters in this source show for the first time very extended thermal water vapour emission. We have detected lines up to 1000 K and a few lines with upper energy levels around 2000 K. These rotational transitions involving high energy levels appear in absorption. We would like to analyze the excitation conditions of water vapour in Orion-IRC2 by observing 35 additional lines of water vapour and 27 of H2(18)O. The first analysis we have carried out show that H2O is very optically thick. Nevertheless, the observed fluxes are an excelent indication of the excitation conditions (Temperature, density). We also propose to map four lines of H2O and two lines of H2(18)O. The proposed observations will provide a major contribution from ISO to our knowledge of the role of water in the chemistry and physics of the interstellar medium.