Direct evidence revealing fundamental differences in sequence of phase formation during the growth of TiSi2 in the presence of an ultrathin surface or interface Mo layer is presented. Results of cross-sectional transmission electron microscopy showed that when the Mo layer was present at the interface between Ti films and Si substrates, C40 (Mo,Ti)Si2 formed at the interface, and Ti5Si3 grew on top after annealing at 550 °C. Additionally, both C54 and C40 TiSi2 were found in the close vicinity of the C40 (Mo,Ti)Si2 grains. No C49 grains were detected. Raising the annealing temperature to 600 °C led to the formation of C54 TiSi2 at the expense of Ti5Si3, and the interfacial C40 (Mo,Ti)Si2 also began to transform into C54 (Mo,Ti)Si2 at 600 °C. When the Mo was deposited on top of Ti, the silicide film was almost solely composed of C49 TiSi2 at 600 °C. However, a small amount of (Mo,Ti)5Si3 was still present in the vicinity of the sample surface. Upon annealing at 650 °C, only the C54 phase was found throughout the entire TiSi2 layer with a surface (Mo,Ti)Si2 on top of TiSi2. Hence, it was unambiguously shown that in the presence of surface versus interface Mo, different routes were taken to the formation of C54 TiSi2.