Significant progress in our knowledge of the Pluto-Charon system
 has been obtained in the last decade. In particular, it was
 discovered that (i) Pluto's surface is not uniform, but presents
 variations in albedo (ii) methane, nitrogen and carbon monoxide
 ices are present at Pluto's surface (iii) Pluto possesses an
 atmosphere of several microbars. From this set of outstanding results,
 a general picture of Pluto's surface and atmosphere and their
 interaction is beginning to appear. However ,a basic piece of the
 puzzle, namely the knowledge of the surface temperature and its
 possible spatial variations, is still essentially missing. The surface
 temperature is a key parameter because it controls the vapor pressure
 equilibrium of the volatiles, thereby defining the atmospheric
 composition, which remains quantitatively unknown. Presently available
 surface temperature measurements, from IRAS (60 and 100 microns) and
 from submillimeter observations, give apparently inconsistent results
 (T = 53-58 K and T = 30-45 K respectively). The contradiction may be
 due to emissivity effects or to a strongly non-uniform surface
 temperature. This issue can be observationnally addressed in three
 different ways (i) repeat the IRAS measurements and extend them to
 shorter wavelengths, where the difference between "warm" and "cold"
 surface will appear even more distinctly (ii) measure the far-infrared
 spectrum (50-200 microns) in narrow photometric bands, allowing to
 study possible spectral emissivity effects (iii) repeat broad band
 photometric measurements to study variations with Pluto's orbital
 phase. A satellite like ISO is obviously required given the wavelength
 range that must be probed and the faintness of the source. We expect
 to obtain a fairly complete description of Pluto's surface temperature
 distribution and thermal properties, that will allow us to firmly
 address such basic issues as the atmospheric composition and vertical
 structure, and the transport of volatiles across Pluto's surface.