Fragment distributions resulting from Au+Au collisions at an incident energy of E/A = 600 MeV are studied. From the measured fragment and neutron distributions the mass and the excitation energy of the decaying prefragments were determined. A temperature scale was derived from observed yield ratios of He and Li isotopes. The relation between this isotope temperature and the excitation energy of the system exhibits a behavior which is expected for a phase transition. The nuclear vapor regime takes over at an excitation energy of 10 MeV per nucleon, a temperature of 5 MeV, and may be characterized by a density of 0.15-0.3 normal nuclear density.