The initial stages of diamond growth on treated Si(001) substrates were studied using various structural and spectroscopic characterization techniques. The nucleation behaviour across the substrate appeared to be inconsistent. Cross-sectional transmission electron microscopy (TEM) revealed the presence of localized amorphous carbon in areas between the Si substrate and the subsequently deposited polycrystalline diamond film, whereas diamond particles, apparently in direct contact with the Si substrate, were observed in other areas. Electron energy loss spectroscopy (EELS) revealed the presence of amorphous carbon in large pockets between polycrystalline diamond particles, as well as in some voids between the Si substrate and the diamond layer. Electron energy loss near edge structure (ELNES) results obtained from a small pocket of amorphous carbon between diamond particles showed a fine structure essentially similar to that of diamond; however, additional intensity was observed in the region ofπ-resonance, indicating mixed sp2/sp3bonding. Although not detected by TEM or EELS, X-ray photo-spectroscopy revealed the emergence of a SIC layer during the early stages of growth.