From the Voyager 1 mission in 1980 and from gound-based observations
 to date, Titan's atmosphere has been revealed to us in terms of
 temperature and composition. The main consituent is N2, followed
 by CH4 and H2. N2+CH4 photochemistry leads to the formation of other
 gas molecules found in trace amounts (hydrocarbons, nitriles and
 oxygen compounds). CO2 was discovered in the infrared Voyager data
 with a stratospheric abundance of the order of 10-8 - 10-9. CO was
 detected from ground-based near-infrared spectroscopy in 1986 with a
 tropospheric mole fraction of about 5 x 10-5. In 1988 a stratospheric
 mixing ratio as low as 4 x 10-6 was derived from millimeter heterodyne
 observations leading to the suggestion that CO may be depleted in the
 stratosphere with respect to the troposphere, the transition occuring
 somewhere between 20 and 60 km. This depletion is not expected due to
 the very long photochemical lifetime of CO. The fates of CO and CO2
 are tightly linked according to photochemical models. To reproduce
 the CO2 Voyager abundance, a CO mole fraction of 1.1x10-4 is required,
 marginally consistent with the near-IR value in the troposphere, but
 disagreeing with the  millimeter CO abundance. Are the CO2 or the CO
 measurements wrong ?  To discriminate between the two, the vertical
 profiles of CO and CO2 need to be determined with high precision.
 CO presents no lines in the thermal IR range and its rotational
 lines in the submillimeter range are too weak to be detectable by
 ISO/LWS. On the other hand, ISO/SWS Fabry-Perot mode offers the
 opportunity to observe the CO2 emission band centered at 667 cm-1
 with great precision: in this range, the spectral resolution achieved
 by ISO is about 215 times higher than Voyager and permits to resolve
 the band, thus yielding information on the CO2 abundance at different
 atmospheric levels. The CO2 vertical profile will in turn constrain
 the photochemical models from which the CO stratospheric abundance can
 then be derived thus solving the problem of its distribution in Titan.