Diamondlike carbon films produced both by ion‐beam technique and by radio‐frequency (rf) plasma decomposition of hydrocarbon gases (C4H10, C2H6, C3H8, and CH4) have been examined using the technique of transmission electron microscopy. Although these examinations indicate that these films are predominantly amorphous, both single‐crystal and polycrystalline diffraction patterns have been obtained from films of both types that indicate formation of several different phases. Some of these phases appear to be cubic and could be new forms of carbon. The results of secondary ion mass spectrometric analysis of carbon films produced by rf plasma decomposition of hydrocarbon gases are also discussed.