The coating structure and barrier property relation of coated PET films is of high interest for understanding the difference between predicted and actual barrier performance of coated PET films. hi this work the chemical and morphological structure of HMDSO coatings generated with and without oxygen in a microwave plasma has been investigated with XPS and AFM analysis. These results were correlated with oxygen permeation measurements at different strain rates and temperatures. The results show that with a more glass-like chemical composition the barrier property is improved. The growth of the coatings takes place in a columnar manner and the interfaces between the columns seem to be the low energy passages for the permeation, which cause the difference between predicted and actual barrier performance. When the coatings are strained the barrier fails at a strain higher than 1%. Cracks occur and with higher strain rates the number of cracks increases. Cracking takes place perpendicular to the strain direction and at the interfaces of the columns of the coating. (c) 2005 Wiley Periodicals, Inc.