Microstructural information obtained for silicon-alloyed pyrolytic carbon coatings used for heart valves using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy is described. The coatings consist of two phases, pyrolytic carbon and β silicon carbide, whose crystallite sizes are small. The coatings are principally composed of spherical particles that are from 300 to 1000 nm in diameter. These spherical particles, between which there are occasionally pores, are welded by laminar carbons. The silicon, content of which is moderate, disperses uniformly in the coatings.