The phase diagram of nuclear matter is quite rich - it shows suchphenomena as phase-transitions, formation of condensates, clustering, etc.From the analysis of the spinodal instability, one can learn about the region ofliquid-gas coexistence in nuclear matter at low densitiesand finite isospin asymmetries. In a recent paper, we have shownthat asymmetric nuclear matter at sub-nuclear densities shouldundergo only one type of instability.The associated order parameter is dominated by the isoscalar density and sothe transition is of liquid-gas type. The instability goes in the directionof a restoration of the isospin symmetry leading to a fractionationphenomenon.