This paper is devoted to a comprehensive study of the photo-oxidation of polymeric materials with the goal of correlating modifications of the polymer properties at the molecular and macroscopic levels. Several techniques were used to characterise the modifications of the chemical properties and mechanical behaviour over time under UV light. The methodology was developed on materials used as organic coatings; initially, a well-characterised phenoxy resin (PKHJ ) was chosen as a model and then the approach was applied to an acrylate-melamine thermoset currently used as a topcoat in the automotive industry. Analysis of degraded samples by IR spectroscopy allowed us to propose a photooxidation mechanism. This mechanism suggested that chain scission occurred under photo-oxidation. To entirely understand the degradation of the polymers, gel fraction, thermoporosimetry, DMA, AFM nanoindentation and micro-hardness determinations were performed. The results showed that crosslinking reactions occurred in competition with chain scission and explained for the first time why crosslinking reactions were quite prevalent. Based on the obtained results, quantitative correlations were made between the various criteria of degradation, thus relating the chemical structure changes to the mechanical property modifications.