[en] It is shown that in many physical situations the spinodal decomposition of unstable systems can be correctly described by stochastic mean-field approaches. Such theories predict that the occurrence of spinodal instability, leading to the multifragmentation of an expended nuclear system, can be signed through the observation of time scales for the fragment formation of the order of 100 fm/c and of typical fragment size around A=20. It is pointed out that these fragments are formed at finite temperature and so can subsequently decay in flight. Possible experimental signals of such first order phase transitions are considered. (authors). 12 refs., 5 figs