The effect of the nitrogen dilution on the optical and vibrational properties of amorphous silicon carbide (a-SiC) and silicon oxycarbide (a-SiCO) layers have been studied. The films were prepared by radio frequency (rf) reactive magnetron sputtering using an atmosphere mixture of argon (Ar) and nitrogen (N2). The oxygen (O2) was incorporated according to the base pressure used of each deposition process. The optical and vibrational properties of the films were characterized by means of UV/VIS transmittance measurements and Fourier transform infrared spectroscopy (FTIR), respectively. A relationship between the variation of the optical bandgap and the increase or quench of vibrational modes is established. This analysis reveals that the increase of nitrogen in both host matrices a-SiC and a-SiCO induced the formation of C=N and C≡N bonds thus reducing the bandgap of the material.