M star spectra beyond 1.35 microns are dominated by water vapour yet terrestial water vapour makes it notoriously difficult to make accurate measurement of the water vapour bands. We propose observations of the strongest water vapour absorption, impossible from terrestial sites, from 2.5--2.9 microns in a range of M stars. ISO provides the resolution to detect ro-vibrational water transitions. We will be able to compare these observations with our predictions based on ab initio calculations of water vapour incorporated into the most sophisticated models for low-mass stars and brown dwarfs. Such comparisons will show up any deficiencies in the model treatment of water vapour at high temperatures and pressures and be essential for the construction of a reliable HR diagram for all O-rich objects below 3500 K (dwarfs and giants). The sensitivity of water vapour absorption strength to temperature, pressure and abundance is expected to yield accurate temperatures, metallicities and gravities for the commonest stars. The depth of the water vapour absorption bands through this spectral region, expected to be around 60\% for VB10 represents a significant uncertainty in the determination of the bolometric luminosity for late-type stars. These observations will remove this uncertainty and enable the bolometric luminosity of late-type stars to be accurately determined. In additional, these observations are expected to play a vital role towards understanding high mass brown dwarfs for which this region will be close to the flux maxima.