Crystallization of rapidly heated samples of Fe 80B 20 has been studied using the methods of electron microscopy, electron diffraction and Mössbauer spectroscopy. Samples have been annealed at temperatures from 733 to 803 K, which is about 100 degrees above the usual crystallization temperatures. We have found that at temperatures around 733 K supersaturated solid solution of boron in α-Fe, Fe 4B, is formed. With increasing time or temperature of annealing tetragonal Fe 3B, orthorhombic Fe 3B and tetragonal Fe 2B are formed, respectively. The morphology of these phases remains spherulitic. Mössbauer spectroscopy has proven that the initial stage of crystallization is characterised by tetragonality similar to the martensitic α-FeC lattice.