A large fraction of the interstellar oxygen inventory is currently
 unaccounted for.  Based on both the results of laboratory studies and
 theoretical models it is clear that one possible reservoir for the
 missing oxygen may be in the form of ozone (O3), a molecule with a
 number of allowed infrared transitions in the 4 to 15 micron spectral
 region.  In dense molecular clouds the ozone is expected to be present
 in the solid form, i.e. as an ice.  Detection of interstellar ozone
 ices would be an important advance since it would not only account for
 some of the missing oxygen but would also provide a great deal of
 information about the conditions and physical/chemical processes
 occuring in interstellar dense molecular clouds.  Atmospheric ozone
 makes it impossible to search for interstellar ozone using ground-
 based and airborne telescopes.  A spaceborne platform is essential to
 probe for this potentially very important carrier of oxygen.  This
 proposal requests ISO time to search for spectral evidence of ozone-
 containing ices in interstellar dense molecular clouds.  Studies of
 ozone will provide insight into O-rich environments and, along with
 CO2 observations being made as part of the GTO program by D. Whittet,
 will give a good picture of the disposition of oxygen inside dense
 molecular clouds.