A silicon oxynitride (SiOxNy) thin film was deposited on polyether sulfone (PES) as a barrier layer against water vapor permeation using reactive radio frequency (RF) magnetron sputtering with a pure Si target in an Ar/N-2 atmosphere. The coating parameters studied included RF power, N-2 content in atmosphere and substrate bias. The water vapor transmission rate, thickness, chemical bonds, microstructure and light transmittance of the films were measured. Taguchi analysis shows that the N-2 content has the most significant influence on the permeability of the gas barrier films. Experimental results show that using a fixed working chamber pressure of 1.6 Pa and deposition time of 30 min results in the lowest water vapor transmission rate which is two orders of magnitude smaller than that of uncoated PES; this result was obtained with an RF power of 250W, N-2 content of 100% and without applying substrate bias. By precisely adjusting the N-2/Ar flow ratio, the largest light transmittance was obtained, at 95% N-2. It was also found that the gas barrier properties of the SiOxNy film are heavily influenced by its microstructure. (C) 2015 Elsevier B.V. All rights reserved.