The consequences of the photodegradation of celluloid on the modification of the material properties have been analysed. Various techniques were used to follow the accelerated degradation provoked by exposure to light (λ > 300 nm). The objective was to relate the modification of the properties at various levels of investigation, from the molecular properties towards the degradation of the macroscopic properties. The modification of the chemical structure was followed by infrared and UV–visible spectroscopic analyses. Gloss and colour measurements were performed, as well as characterization of surface mechanical properties by microhardness and Atomic Force Microscopy (AFM). The results show that the loss of gloss and the decrease of the mechanical properties are a result of a combination of the progressive loss of plasticizer and chain-scission reactions. These developments were explained in light of the modifications of the chemical structure. Quantitative cross-correlations of the degradation chemistry with performance-critical physical properties were established.