In this work we presented the correlation between the chemical composition of amorphous Si:C:N:H layers of various content of silicon, carbon and nitrogen, and their band gap. The series of amorphous Si:C:N:H layers were obtained by plasma assisted chemical vapour deposition method in which plasma was generated by RF (13.56 MHz, 300W) and MW (2.45 GHz, 2 kW) onto monocrystalline silicon Si(001) and borosilicate glass. Structural studies were based on FTIR transmission spectrum registered within wavenumbers 400-4000 cm(-1). The presence of Si-C, Si-N, C-N, C=N, C=C, C equivalent to N, Si-H and C-H bonds was shown. The values band gap of the layers have been determined from spectrophotometric and ellipsometric measurements. The respective values are contained in the range between 1.64 eV - characteristic for typical semiconductor and 4.21 eV - for good dielectric, depending on the chemical composition and atomic structure of the layers. (C) 2016 Elsevier B.V. All rights reserved.