The chemistry of HfI 4 adsorbed on the Si(100)-(2×1) surface has been studied by core level photoelectron spectroscopy in ultra-high vacuum. Two stable surface intermediates are identified: HfI 3 and HfI 2 , both of which remain upon heating to 690K. The dissociation of HfI 4 is accompanied by the formation of SiI. In addition, HfI 4 is observed up to 300K. Complete desorption of iodine occurs in the temperature regime 690–780K. Deposition of HfI 4 at 870K results in a layer consisting of metallic Hf, whereas deposition at 1120K results in the formation of Hf silicide. The results indicate that the metallic Hf formed at 870K is in the form of particles. Oxidation of this film by O 2 at low pressure does not result in complete Hf oxidation. This suggests that complete oxidation of Hf is a critical step when using HfI 4 as precursor in atomic layer deposition.