The structural evolution from amorphous to crystalline mullite, for different 3Al(2)O(3) . 2SiO(2) mono- and diphasic precursors, has been investigated by Si-29 and Al-27 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The crystallization has also been studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chemical composition in the aluminosilicate network of the diphasic precursors and in the crystallized phases has been determined from the Si-29 NMR spectra. A close agreement is found with the composition deduced from the lattice parameters measured by XRD. For monophasic precursors the amount of hexa-coordinated aluminium atoms decreases when the temperature increases while AI(IV) and AI(V) increase. AI(VI) practically completely disappears just before the crystallization at 980 degrees C. An alumina-rich mullite 2Al(2)O(3) . SiO2 (2:1 mullite) is then formed through a strong exotherm. An enthalpy of 75 kJ per mot is determined for the crystallization of the 2:1 mullite. At higher temperatures the segregated silica is progressively reincorporated into the mullite lattice. For diphasic precursors the Si-29 NMR spectroscopy shows the segregation of silica. The aluminosilicate network is then richer in alumina and the amount of remaining AlO6 octahedra before the crystallization at 980 degrees C is higher. Spinel crystallizes and continues to become richer in alumina until it reacts with silica to form the stoichiometric 3:2 mullite at 1260-1275 degrees C. The nature of the crystallization is related to the local composition of the amorphous alumino-silicate network and to the amount of AlO6 octahedra on approaching 980 degrees C.