 Photocatalyst activity indicator inks ("Paii's") are a recent advance in the visual demonstration of photocatalysis and the assessment of the activity of photocatalyst materials.They are an inexpensive, easy to use and provide a very quick route to demonstrating the presence of a photocatalytic film, even under low levels of UV light.Unlike the photo-oxidative bleaching of methylene blue, they use the underlying semiconductor photocatalyst film to photoreduce the dye (Dox in figure 2), in the ink coating, to another (usually colourless) form, (Dred in figure 2) whilst simultaneously oxidising an easily oxidised organic species, a sacrificial electron donor (SED), such as glycerol, which is also present in the ink.