The hydrothermal behavior of α-SiC crystals has been studied in the temperature range 500–700 °C under 30–200 MPa pressure at different pH values. The extent of carbon formation via selective oxidative extraction of Si out of the SiC crystal lattice and the structure of the carbon formed depends on the experimental conditions. Homogeneous carbon coatings were obtained on machined crystal surfaces at ca. 600 °C and p ≥ 60 MPa using distilled water. They were analysed using TEM, SEM, optical microscopy, micro-Raman spectroscopy, photoelectron spectroscopy, Auger spectroscopy and SIMS. The films consist of hydrogenated carbon which contains up to 13 atom% oxygen on the surface and ca. 3 atom% oxygen in the bulk. Predominantly amorphous sp²-carbon formed, which shows little tendency to form turbostratic disordered or partly crystallized graphite. However, some particles containing nanocrystalline diamond were also found. A mechanism for hydrothermal carbon formation is proposed. The different methods for carbon analysis applied are compared.