Thin silicon oxide (SiOx) barrier coatings formed by plasma enhanced chemical vapor deposition on poly(ethylene terephthalate) (PET) substrates were subjected to post-deposition annealing treatments in the temperature range for orientation relaxation of the polymer. The resulting change in coating internal stress state was measured by means of thermo-mechanical analyses, and its effect on the coating cohesive properties and coating/polymer adhesion was determined from the analysis of uniaxial fragmentation tests in situ in a scanning electron microscope, assuming a Weibull-type probability of failure and a perfectly plastic stress transfer at the SiOx/PET interface. The strain to failure and intrinsic fracture toughness of the ultrathin oxide coating were found to be as high as 5.7% and 10 J/m2, respectively, and its interfacial shear strength with PET was found to be close to 100 MPa. Annealing for 10 min at 150 -C did not modify the oxygen permeation properties of the SiOx/PET film, which suggests that the defect population of the oxide was not affected by the thermal treatment. In contrast, the coating internal compressive stress resulting from annealing was shown to increase by 40% the apparent coating cohesive properties and adhesion to the polymer.