Deals with the process of photooxidation weathering of a film of Rembrandt Picture Varnish Series 3 produced by Talens on the basis of cyclohexanone resin. Experiments were performed in ambient atmosphere at 20° C and 50% relative humidity in a climatic chamber, Feutron 3001, provided with a medium pressure mercury radiator and a short wave (less than 290 nm) filter. The effects of mass increase of the varnish layers, the change of solubility, hardness, and absorption in infrared indicate photooxidation to be the cause of yellowing and dullness, loss of solubility in white spirit, and the increase in hardness and brittleness. No data were obtained that could indicate this process was accompanied by photolysis or cross-linking.