  We investigate the thermodynamical properties of nuclear matter in heavy ion
collisions, in particular with respect to questions of thermodynamical
instability, phase transitions and fragmentation. For this we analyze results
of relativistic transport calculations of Au+Au collisons at intermediate
energies, separately for spectator and participant matter. On one hand, we
determine local thermodynamical variables from the analysis of the local
momentum distribution; on the other, we analyze fragment energy spectra in a
blast model scenario. We find that the spectator represents an instable,
equilibrized fragmenting source, while in the participant no such common source
can be identified. Our results compare well with experimental determinations of
temperatures and flow velocities.
