Diamond-like carbon (DLC) coatings are promising candidates for improving the mechanical wear and chemical resistance of load-bearing implants. However, in view of medical data showing severe problems due to film delamination, the application of DLC layers to the medicotechnical field is not yet recognized. In an extension to an earlier publication, in which we established that coating delamination in phosphate-buffered saline (PBS) solution is driven by stress–corrosion cracking, we present for the first time a detailed structural and chemical analysis of the interface region of DLC-coated CoCrMo. High-resolution and scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy shows that a 5 nm thick carbide layer with an average Me:C stoichiometry of 2:1 is formed at this interface. It is shown that the failure of DLC-coated CoCrMo in PBS is due to the instability of Co carbides at the film–implant interface.