A mathematical model was derived for the oxidation behaviour of a polymer film, taking into account both diffusion free and diffusion controlled kinetics. Comparison with experimental results showed excellent agreement. Expressions were derived from this model for the reaction induction time, and the limiting thickness of film at which diffusion effects become important. Investigation of the photostabilisation mechanism of Hindered Amine Light Stabilisers was undertaken. After experimental investigation of numerous proposed stabilisation processes, and their rejection, a reaction scheme was proposed involving the scavenging of trace transition metal ions in the polymer matrix.