The reaction between sputtered Ti thin films and heavily arsenic doped Si(100) is studied. The use of an arsenic implantation to pre-amorphize the Si substrate and the choice of the substrate temperature during Ti sputtering are both found to have a significant effect on subsequent TiSi2 reactions. Cross-sectional transmission electron microscopy reveals that an amorphous TiSix layer is formed at the interface between Si and as-sputtered Ti. The thickness of this interfacial layer increases with the sputtering temperature. After rapid thermal anneals in nitrogen, the sheet resistances of TiSi2 thin films grown with the pre-amorphization step and a high sputtering temperature (450°C) are generally lower than films processed under other conditions. This apparent reduction in the temperature for the polymorphic C49 to 54 phase transformation in TiSi2 is shown to originate from a higher nucleation density of the C54-TiSi2 phase. These dependencies of the silicide reaction are ascribed to the interfacial amorphous TiSix layer. In increasing the nucleation density of the C54-TiSi2 phase, the amorphous TiSix layer is speculated to either act as a direct nucleation source for the C54-TiSi2 phase, or lead to more defective C49-TiSi2 structures which facilitate the C54-TiSi2 nucleation.