Amorphous boron carbide (a‐B1−xCx) is believed to have an icosahedron‐based random network. In this paper, vibrational properties ofa‐B1−xCxfilms are studied by IR and Raman spectra, placing particular emphasis on the interpretation of the most prominent 1100‐cm−1band associated with the B–C bond. The 1100‐cm−1band appears in both IR and Raman spectra, and the frequency variation and the intensity as a function of the C content are examined, together with evaluation of the absolute absorption coefficients. Within the framework of the impurity‐induced vibration theory, the 1100‐cm−1band is characterized. The estimation of the force constants by the observed frequencies leads to a conclusion that the vibration can be classified as an extrinsic and preferably a local mode. This vibration is well described as the stretching mode of a localized two‐center bond between the B and C atoms. In this sense, the C atom in amorphous B1−xCxis not regarded as a network constituent. The frequency shift with the C content supports this conclusion. The absorption intensity of this band exhibits large values for the transverse effective charge ofa‐B1−xCx. A numeric estimation shows that most part of the observed transverse effective charge comes from the localized charges of the C impurity. This mechanism of the effective charge is consistent with the above argument of frequency. The intrinsic vibrations of the amorphous boron network based on the icosahedron structure reside over the range from 400 to 1000 cm−1with a small peak at 800 cm−1. ©1995 American Institute of Physics.