The mechanical properties of ultrathin silicon oxide (SiOx ) coatings plasma-deposited on poly(ethylene terephthalate) (PET) films were investigated with particular attention paid to the effect of additives located in the superficial layers of the polymer substrate. The cohesive and adhesive properties of the thin oxide coating were derived from the analysis of fragmentation tests carried out in situ in a scanning electron microscope. The cohesive strength of the coating was determined assuming a Weibull probability of failure of the oxide, and the coating/substrate interfacial shear strength (IFSS) was calculated by means of a stress transfer analysis with a perfectly plastic interface. It was shown that the presence of additives in the superficial layers of PET substrates leads to a 20% decrease of the crack onset strain, which is due to an increase of the coating defect density, as revealed by means of atomic oxygen etching. The stress concentration induced by coating microdefects was modeled, and was shown to induce a decrease in the cohesive properties of the coating, which correlates with the observed decrease of crack onset strain. Moreover, the adhesion was found to be very high, with a IFSS higher than the substrate bulk shear stress at yield, irrespective of the presence of additives.