Photocatalyst activity indicator inks are a recent advance in the visual demonstration of photocatalysis and the assessment of the activity of photocatalyst materials. They are 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 (D in figure 2), in the ink coating, to another (usually colourless) form, (D 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. The kinetics of reduction of the dye in a paii have been studied in great detail. Figure 2 illustrates the basic principles of operation of a paii when applied to a product that has a thin photocatalyst film coating.