In this work, low frequency PECVD a-SiCxNy:H thin films have been synthesized in the temperature range 25-300 A degrees C from hexamethyldisilazane precursor mixed with ammonia at various concentrations. A relevant correlation has been evidenced between the [N]/[C] atomic ratio in the gaseous phase and in the deposited thin films, allowing both prediction and control of the film microstructure. A simple method based on the analysis of the films FTIR spectra was proposed to determine the value of the [N]/[C] ratio and thus predict or adjust the gas transport properties of the membrane materials. Attractive ideal selectivities alpha*(He/N2) exceeding 90 with He permeance I (He) > 3.10(-7) mol.s(-1).m(-2).Pa-1 were measured at 150 A degrees C for the films prepared at 300 A degrees C with an optimum [N]/[C] atomic ratio in the range 0.1-1.5. These films behave as molecular sieve membranes with a thermally activated transport of helium.