The study of comets is of great importance to our understanding of the
 solar system and its origin.  Comets comprise the most primitive
 material in the solar system and observations of their composition
 are diagnostic of the environment in which they formed.  In recent
 taxonomic classification of comets, we have clearly separated out the
 majority (~70%) of comets having "typical" molecular abundances from
 a second distinct class of comets that exhibit significant depletions
 in the carbon-chain species (C2 and C3).  The ISO Guaranteed Time
 targets; P/Kopff, P/Churyumov-Gerasimenko, and P/Wild 2 are all
 members of the "typical" class of comets.  The "carbon-chain depleted"
 class of comets must be studied in similar detail as those of the
 "typical" class if we are indeed to gain a firm understanding of the
 processes that took place during the early solar system.

 Two of the three most extreme members of the the "carbon-chain
 depleted" class are positioned favorably for observations by ISO.
 With an autumn launch, comet P/IRAS would be the target.  For a
 spring launch, the target would be P/Wolf-Harrington, the most
 extreme member of this class, with P/IRAS serving as a secondary
 target.  These targets have gas and dust production levels
 approximately a factor of 3-5 below that of the principal GTO target,
 P/Kopff and about 1% the production of P/Halley.  Results from this
 observational programme are highly complementary to the Guaranteed
 Time Observations of comets [PI, E.Gruen].