The adhesion between different types of polyethylene terephthalate (PET) substrates coated with thin silicon oxide (SiOx) layers is examined using two alternative approaches. The surface of the polymer was hydrolyzed or silylated prior to the deposition of the oxide layer, to be compared with untreated PET. The first approach is the thermodynamic adsorption model from which are defined the dispersive and polar components of the polymer surface energy, obtained from wetting measurements. The second approach is the micromechanical analysis of the interface stress transfer which provides the interface shear strength from the measurement of the density of coating cracks vs. applied tensile strain. The hydrolysis treatment slightly hydrophobizes the PET surface; however, it does not significantly modify the interface shear strength compared with the untreated material. By contrast, the silane treatment improves the polar component of the PET, which is related, to a first approximation, to the measured 30% increase of the interface shear strength compared to the untreated material.