The observation of deuterium fractionation in hot molecular cores is
currently interpreted as evaporation of deuterium-rich molecules from
the icy grain mantles where they accummulated during the cold, dense
cloud core stage. 
Presently, however, that enrichment in deuterium of the ices, even
though theoretically predicted, has never been observed.
We therefore propose to measure the D/H ratio in water-ices in grain 
mantles towards a young stellar object embedded in a dense cloud, by 
observing the O-D stretch of HDO at 4.08 micrometers. Its detection will 
provide the missing link between theory and observations in dark cloud 
chemistry.
For the observation of the O-D stretch it is necessary to take a spectrum
from ~3.6 to ~4.4 micrometers, the long-wavelength part of which is
inaccessible from the ground, therefore the need for ISO data.