Backscattering of He ions and Seemann-Bohlin x-ray diffraction techniques have been used to investigate the composition and growth kinetics of hafnium silicide formed from thin films of hafnium sputter deposited on silicon. The stable-phase HfSi was formed in the temperature region 550-750°C. Zirconium and argon impurities were present in the Hf films at the level of 2.5 at.%. During the formation of the HfSi phase, these impurities served as markers which were used to determine the diffusing species. We conclude that Si is the diffusing species.