The composite oxygen-barrier coating of plasma polymerized-hexamethyldisiloxane (pp-HMDSO) and silicon oxide (SiOx) on polypropylene (PP) was performed to lower the oxygen transmission rate. A pp-HMDSO interlayer was employed to cover PP pores at a relatively high porosity and provide better a thermal and mechanical buffer between brittle SiOx and soft and porous PP. There is an optimum thickness for the pp-HMDSO and SiOx films due to competition between the accumulated strain energy by compression in the composite layers and the interfacial adhesion strength. By choosing the optimal thickness of the layers, the oxygen transmission rate was improved from 7.42 x 10(-4) for a pristine PP container to 2.6 x 10(-5) cm(3)/m(2)-day-Pa with 8 +/- 2 nm pp-HMDSO and 24 +/- 6 nm SiOx layers. (C) 2013 Elsevier B.V. All rights reserved.