We propose to obtain low resolution mid-IR spectra of a
 sample spanning the temperature range of the coolest
 known M-dwarfs (Teff = 3000K - 2000K). ISO is UNIQUELY
 suited for obtaining these observations, as the features of
 most interest in the mid-IR spectra of these stars (H20, CO, CO2
 CH4) fall in the middle of the wavelength regions made
 inaccesible by the Earth's atmosphere. These observations, made
 for the first time in the spectral range of the atmospheric
 `walls', will provide essential new data for the next generation
 of atmospheric models. This will allow them to be brought in line
 for the first time with observations -- allowing a meaningful Teff
 scale to be constructed for the faintest M-dwarfs, and more
 importantly, for brown dwarf candidates.