Amorphous carbon nitride (CNx) films were prepared in CH4/N-2 r.f. plasma, and were examined by means of IR absorption spectroscopy. The IR absorption bands associated with N-H (3200-3500 cm(-1)), C-H (2800-3000 cm(-1)), C=N (2100-2250 cm(-1)), and N-H and/or C=N (1500-1800 cm(-1)) bonds, were observed. The IR absorption band at 2100-2250 cm(-1) region was examined in more detail, and accordingly it was confirmed that this band contained five absorption peaks at around 2105, 2160, 2190, 2215 and 2245 cm(-1), which could be assigned to nitrile (-C=N) (2215 and 2245 cm(-1)) and isonitrile (-N=C) (2105, 2160 and 2190 cm(-1)) structures, respectively.