We propose to observe four key transitions of the H2O molecule connected to the 6(16) and 5(23) levels, which give rise to the prominent and well observed 22 GHz (1.3 cm) maser line in circumstellar shells of variable stars on the AGB. Most of these transitions emit photons with energies in the FIR and part of them are within the LWS range and can be observed with a Fabry-Perot line scan. Such observations will probe current models for the maser pump. The most elaborated model by Cooke and Elitzur (1985) predict pumping by colli- sions and on this base an increase of maser luminosity and distance from the star with increasing mass loss rate. This is confirmed as general trend observationally. However,observations tell that individual stars with similar mass loss rates can have quite different maser luminosities and line profiles. Either the pumping conditions are different in these stars or the velocity structure of the outflowing material is different. Observa- tions reveal also that the maser is strongly variable in response to variations of the luminosity of the central star. To probe the influence of the pumping conditions on the observed maser properties, we selected stars representing at first extremes in the range of mass loss rates and at second extremes in the range of maser luminosities. Observations at fixed times of their light curves will allow to study the variations of line strength in the FIR. We want to probe our suspicion that the extinction of H2O maser emission around minimum light in many OH/IR stars is due to a combination of quenching and insufficient excitation temperatures. We request addition of ISOPHOT photometry in the IRAS 12,25, 60 micron bands at each time of an LWS04 observation. With additional NIR data the infrared energy distribution obtained by SWS and LWS grating scans within the Guar. Time Prg. can be scaled to the date of the LWS04 ob- servation. This will allow comparison of dust and gas temperature.