Wedemonstrate significantly enhanced barrier properties of polyetheretherketone (PEEK) against water vapour penetration by depositing Al2O3 or Al2O3/TiO2 nanofilms grown by atomic layer deposition (ALD). Nanoindentation analysis revealed good adhesion strength of a bilayer Al2O3/TiO2 coating to PEEK, while the single layer Al2O3 coating displayed flaking and delamination. We identified three critical design parameters for achieving the optimum barrier properties of ALDAl(2)O(3)/TiO2 coatings on PEEK. These are a minimum total thickness dependent on the required water vapour transmission rate, the use of an Al2O3/TiO2 bilayer coating and the application of the coating to both sides of the PEEK film. Using these design parameters, we achieved a reduction in moisture permeability of PEEK of over two orders of magnitude while maintaining good adhesion strength of the polymer-thin film system. (C) 2015 Elsevier B.V. All rights reserved.