The optical emission spectroscopy has been used to study the decomposition process of tetramethylsilane in a low pressure plasma. The Si(CH3)4–N2–H2 and Si(CH3)4–N2–Ar reactive mixtures, which are used for the deposition of SiCN layers, have been studied here. High energy active species were identified in the plasma phase and the electron excitation, vibrational and rotational temperatures as well as electron number density were determined for various compositions of the reactive mixtures. The electron excitation temperature (Si I, Ar I, H) was found to be higher than the vibrational temperatures (CN, N2, N2+) and considerably higher than the N2+ rotational temperature, as the results of nonequilibrium state of plasma generated. It was observed that introduction of tetramethylsilane as well as growth of hydrogen percentage led to lowering of the electron density and the rotational temperature. Optical actinometry was applied to study the Si(CH3)4–N2–H2 reactive mixture.