Initial stage of the C49–TiSi2formation was investigated at 530 °C and at a rate of 10 °C/m using transmission electron microscopy. Morphological studies reveal that the C49 phase first separately nucleates at the interface between amorphous silicide and crystalline silicon, then followed by simultaneous lateral and vertical growth. The growth proceeds very fast until the formation of a continuous layer of C49–TiSi2. Local chemical analysis shows that the composition range of the amorphous silicide is narrowed due to the C49 formation. For isothermal annealing, a linear density of the C49 nuclei is about 6.7×10−3/A˚, and remains the same upon prolonged annealing. In the case of annealing at 10 °C/m, the linear density depends on temperature, reaching a maximum of 7.2×10−3/A˚ at around 575 °C.