The microstructure and kinetics of the polymorphic C49 to C54-TiSi, phase transformation have been studied using samples prepared as in self-aligned silicide applications. For C49-TiSi, thin films formed at temperatures of 600 and 625-degrees-C on (100) single-crystal silicon substrates, the effective activation energy was 5.6 +/- 0.3 and 5.7 +/- 0.08 eV, respectively, for the C49 to C54 phase transformation carried out in the temperature range 600 to 700-degrees-C. We concluded that the transformation process occurred by nucleation and growth of the orthorhombic face-centered (C54) phase from the as-formed orthorhombic base-centered (C49) phase. The Avrami exponent of 2.2 +/- 0.09 and the optical observations suggest that most of the nucleation occurred during the beginning of the transformation process.