We studied the nucleation and growth of nano-sized crystals on two glass-ceramic systems: a conventional lithium-aluminosilicate (LAS) and a (Mg,Zn) spinel. We combined several techniques: in situ Small Angle Neutron Scattering (SANS), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), and laboratory X-ray diffraction (XRD). We observed by SANS, and confirmed by DSC, that during a temperature ramp, transient phenomena occur between the regions of nucleation and growth in the LAS, which do not follow classic kinetic theories. In contrast, the spinel material shows a smooth transition during the temperature ramp between the nucleation and the growth stages, and follows a more conventional growth pattern. In the spinel system the initial phase separation plays a very important role in determining the crystalline phase distribution in the glassy matrix, as crystallites are confined only in one phase.W. Lee-contributing editor Manuscript No. 30287. J ournal aluminosilicate nucleated by TiO 2 alone. In both cases nucleation and crystallization mechanisms appear very complex, and involve three-phase immiscibility. In magnesium aluminosilicate, spinel crystallites form together with magnesium-aluminum-titanate. In zinc aluminosilicate the crystalline phases are gahnite (Zn-spinel) and two metastable forms of TiO 2 : anatase and brookite. Incidentally, we notice that these mechanisms are different than that observed in LAS glassceramics nucleated by TiO 2 alone, studied in Ref. (15). h