Amorphous Si/Ti multilayers transform at high temperatures (above 700°C) to TiSi2-C54. This phase is important for microelectronics applications because of its low resistivity, stability up to 900°C, and compatibility with silicon processing. However, an unfavorable metastable TiSi2-C49 phase is usually formed at lower temperatures. Thus, an understanding of the C49 phase formation kinetics is useful to the device processing strategy. The kinetics of the transformation of TiSi2-C49 phase can be characterized as a process of nucleation and growth, using the well known Johnson Mehl Avrami (JMA) equation. In the present work the formation kinetics of the C49 phase has been studied by an in situ x-ray diffraction technique. Isothermal annealing in vacuum was done at four temperatures, in the range of 275"C to 310°C. A position sensitive detector (PSD) was used to simultaneously collect the diffracted beams of (131) and (150) peaks of the C49 phase. From the data, the Avrami exponent, n, was determined to be 2.0±0.1. The reaction rate constant k follows a familiar Arrhenius-type equation with a measured activation energy of 2.5eV. Comparison of our x-ray results with kinetic data obtained by other means will be discussed.