Spin resonance has been used to study the unpaired spins in amorphous carbon films prepared by the plasma decomposition of propane. The properties of amorphous carbon prepared by this method are very diamond like. The ESR signal shows saturation effects and a fairly large positive g shift (g=2.003 ± .0003). Both of these characteristics are associated with unpaired spins in a delocalized system, e.g. aromatic radicals. We conclude that there is appreciable trigonal bonding in this form of amorphous carbon which causes some delocalization into a pi molecular orbital. © 1980.