The hardness of amorphous carbon nitride (CN) was investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FTIR). The results of NMR measurements showed that sp(2) and/or sp hybrid orbitals controlled the hardness of amorphous-CN films, which had been determined by nanoindentation tests. The intensity of the C-H and CN stretching modes observed in the FTIR spectra was the weakest in the hardest sample. The decreases of -CH3 and -CH2 groups indicated that the amorphous network had increased. The sp2 hybrid orbital forms a cross-linker which plays an important role in improving of the hardness of the film. (C) 2004 The Electrochemical Society.