Ultra-thin (10–500 nm) photo-polymerized γ-methacryloxypropyltriethoxysilane (MPMS) and vinyltrimethoxysilane (VTS) layers applied to SiO2 coatings on polyethylene terephthalate films improved the oxygen barrier performance of the oxide coated films by more than two-fold, and the coating strain to failure from 1.5% to beyond 5%, depending on the processing conditions. The oxygen transmission rate of composite films was measured under tensile strain. The chemical conversion of both MPMS and VTS with two different photoinitiators was studied by means of photo-calorimetry experiments, with attention paid to the influence of temperature, UV-light intensity and photoinitiator concentration. The conversion data were analyzed using an nth-order kinetic model, including an Arrhenius dependence of the temperature and a power-law dependence of the light intensity on the reaction rate. The improvement in mechanical integrity of the silane/silica barrier coating, with a toughness as high as 80 J/m2 was found to be controlled by the conversion state of the silane.