1 DESPA - Observatoire de Paris, France
2 ARPEGES - Observatoire de Paris, France
3 Osservatorio Astronomico di Torino, Italy
4 Université ``Denis Diderot'' - Paris VII, France
5 ISO Science Operations Center, ESA, Vilspa, Spain
6 Institute for Astronomy, Univ. of Hawaii, Honolulu, USA
ISO observations of four dark asteroids (albedo < 0.08) have been performed with SWS.
The obtained results in the spectral range 5.3-45 
will be presented
for 10 Hygiea, 114 Kassandra, 308 Polyxo and 624 Hektor.
These objects (diameter between 100 and 400 km) belong to the C and D
taxonomic
type and represent the more primitive objects among the asteroid population.
The C asteroids, similar to carbonaceous
chondrites, are probably unaltered bodies darkened by opaque organic
material
that dominated their reflectance properties. The D class objects are thought
to be even richer in organics and hydrated clay minerals,
unaltered ultracarbonaceous
bodies formed at low temperature in the Solar Nebula.
The compositional investigation of these objects should yield direct information on the early chemical and physical process operating in this portion of the Solar System.