We have studied the structure and composition of the interface of diamond thin films deposited on Si substrates by chemical vapor deposition using atomic resolution electron microscopy, and electron energy loss spectroscopy. The latter technique allows the analysis of the relative Si to C composition as well as the nature of the bonding to determine the relative sp2/sp3 content of the diamond film. Results show considerable intermixing of silicon and carbon across an interfacial region 3 nm thick, which corresponds with an amorphous region. This region also contains the highest concentration of sp2 bonded C. To perform this analysis a novel method of producing diamond TEM samples was developed which combines standard FIB techniques and removal of surface damage by thermal decomposition at 350 °C.