The synthesis of FAU zeolite was studied in situ using scanning microcalorimetry at a slow heating rate of 0.10 degrees C/min. The scanning calorimetric heat flow (SCHF) curves showed characteristic exothermic peaks for both the precrystallization reactions and the crystallization. Combined with complementary characterization techniques, the structural and chemical changes in the zeolite synthesis system were identified and correlated with the SCHF peaks. Two mixtures of the same composition (5.15Na(2)O-1.00Al(2)O(3)-3.28SiO(2)-165H(2)O) but prepared from different silica sources (colloidal silica vs silicate solution) for the syntheses of FAU zeolite were examined. A cyclic or oscillatory pattern for precrystallization reactions was observed for the first time. This precrystallization evolution pattern indicates the stepwise precrystallization reactions that may result in significant inhomogeneity of the Si/Al ratio in the microscopic domains of the amorphous precursor. The existence of the compositional inhomogeneity is consistent with the occurrence of the LTA zeolite impurity (Si/Al = 1.0) when colloidal silica is used but not when solution of dissolved silica is used. A small energetic driving force. for the crystallization (Delta H = -2.1 +/- 0.1 kJ/mol on TO2 basis of the zeolite product) has been directly measured. The integral enthalpy change for the precrystallization reactions varies greatly depending on the nature of the starting materials. The implications of the thermodynamic results in zeolite synthesis are also discussed.