Model experiments using a flow photoreactor (Hgimmersion lamp) connected to a spectrophotometer are useful to study the influence of the state of dye aggregation and of certain addenda on lightfastness. Such experiments can be carried out in the transmission- or in the diffuse reflectance mode (ULBRICHT integrating sphere), the latter being useful for diffuse particulate systems with agglomerates of AlOOH nanoparticles. Such measurements can often be correlated with results in ink-jet receiving layers. Parameters like humidity, temperature, solvents, polymers etc. in ink receiving layers or in inks are shown to influence photolysis by the state of dye aggregation.While we could show that a photoreductive radical mechanism is responsible for certain dyes, photooxidation was observed for other dyes in the same ink receiving layers. The presence of certain dyes in multicolor dots sometimes promotes a triplet-sensitized photocatalytic oxidation which depends on the state of dye aggregation, the chemical structure of the dye and on the presence of certain addenda in ink-receiving layers. Our experiments have shown that no single, well defined mechanism is responsible for the photodegradation of ink-jet dyes. Therefore, no single process is responsible for the photostabilizing action of additives.