This paper reports on the results of the thermodynamic modeling of chemical vapor deposition of SiC _x N _y silicon carbonitride films with the use of the volatile organosilicon compound hexamethylcyclotrisilazane (HMCTS) over a wide temperature range 300–1300 K at low pressures of 10⁻²−10 Torr. It is demonstrated that there are ranges of conditions under which the gas phase is in equilibrium with a mixture of solid phases SiC + Si₃N₄ + C with the total composition represented in the form of the ternary compound SiC _x N _y . Transparent silicon carbonitride films of different compositions are experimentally obtained under conditions in the above range through plasma-enhanced chemical vapor deposition at a pressure of 5 × 10⁻² Torr and temperatures of 373–1023 K with the use of the initial gaseous mixture of hexamethylcyclotrisilazane and helium. The chemical and phase compositions of the films are determined and their properties are investigated using ellipsometry, IR and Raman spectroscopy, spectrophotometry, energy-dispersive spectroscopy, and synchrotron X-ray powder diffraction. It is shown that the films synthesized at low temperatures of 373–573 K contain a considerable amount of hydrogen. The results obtained ftom atomic-force and scanning electron microscopy indicate that the films involve nanograins.