The photo-oxidative stability of a bifunctional epoxy resin has been investigated and compared with that of the same resin additivated with a polyhedral oligomeric silsesquioxane (POSS) functionalized with epoxy groups. The study was aimed at a quantitative assessment of the stabilization effect of the POSS additive and at a deeper understanding of its origin through a detailed description of the photodegradation process both in the pristine and in the POSS-containing resins. Fourier Transform infrared spectroscopy was employed for the kinetic and mechanistic analysis, and dynamic-mechanical spectroscopy to investigate the modifications of the molecular structure of the network. Scanning electron microscopy (SEM) was used for the characterization of the exposed surfaces, and fracture mechanics measurements to evaluate the fracture properties before and after photo-aging. In a homogeneous degradative regime (congruent to 10 mu m films) OG-POSS tends to enhance the sensitivity of the system toward photo-oxidation. On thicker samples (1.0 mm) the general deterioration of viscoelastic properties observed in the plain resin upon photo-degradation is almost completely suppressed with 20 wt% of the additive. A mechanism has been proposed to account for this effect, which was confirmed by SEM analysis of the degraded surfaces. (C) 2012 Elsevier Ltd. All rights reserved.