A multilayered barrier structure was fabricated by chemical vapor deposition of parylene and subsequent plasma-enhanced chemical vapor deposition of SiOx or SiNx. The barrier performance against water vapor ingress was significantly improved by annealing the parylene layer before the deposition of either SiOx or SiNx. The mechanism of this enhancement was investigated using atomic force microscopy, Raman spectroscopy, and X-ray diffraction. The surface roughness of the parylene before the deposition of either SiOx or SiNx was found to correlate closely with the barrier performance of the multilayered structures. In addition, removing absorbed water vapor in the film by annealing results in a lower water vapor transmission rate in the transient region and a longer lag time. Annealing the parylene leads to a large decrease in the effective water vapor transmission rate, which reaches 7.2 +/- 3.0 x 10(-6)g/m(2)/day. (C) 2014 Elsevier Ltd. All rights reserved.