The effect of heat treatment on the glass component for glass ionomer cements (GICs) has been studied, with specific reference to the microstructure, crystallization and phase transformation. The thermal crystallization was characterized by means of scanning electron microscopy, differential scanning calorimetry and X-ray diffraction. Results showed that there was an increase in the particle size after heat treatment. The glass presented three crystallization peaks. The activation energies for crystallization obtained by different analytical methods, namely Kissinger’s, Ozawa’s and Yinnon’s methods, were in good agreement and revealed the propensity of this glass composition to crystallize easily. The crystalline activation energies for the glass determined by Kissinger’s method were 670 and 336 kJ/mol, respectively, associated with the first and second peaks. The calculated Avrami parameters based on the two peaks were 1.58 and 2.97, respectively. The heat treatment produced a final product of strontium fluoride phosphate (Sr5(PO4)3F) and mullite (Al6Si2O13).