stamp, paint brush, or a drawdown bar, and then exposed it to sunlight or an alternative, appropriate light source. The ink identifies the presence of the photocatalyst coating by changing colour upon irradiation of the latter at a rate (usually < 10 min) which provides a measure of the film's activity. For example, it has been established that the rate of change in colour of an paii on commercial self-cleaning glass is directly related to the rate at which the glass is also able to photo-oxidatively mineralise, via reaction (1) the wax-like, natural fatty acid, stearic acid, found in finger prints. The rate of the rapid colour change associated with photocatalyst activity indicator inks has also been directly correlated with the photocatalytic oxidation of methylene blue and NOx. It has also been shown that digital colour analysis of photographs monitoring the colour change of a paii can be used to extract apparent absorbance data which correlates well with UV-vis absorption data for the same sample, without the need for expensive spectrophotometric instrumentation. By making the dyes in the ink increasing difficult to reduce chemically, for example by using: basic blue 66, resazurin, and acid violet 7, respectively, it is possible to make paiis which are effective on photocatalyst coatings which exhibit, respectively: low (most self-cleaning tiles), moderate (self-cleaning glass) or high (self-cleaning paints) activities. paiis based on the dyes 2,6-dichloroindophenol (DCIP) and methylene blue have also been reported. Applications Paiis can be used as quality control and marketing tools in commerce and as a quick and easy way to assess and/or map the activities of new photocatalytic materials in research. In addition, it has also been demonstrated that such inks can be used on highly coloured and black surfaces, provided the oxidised and/or reduced form of the redox dye is luminescent, and that they can be effectively used to demonstrate the activity of visible light photocatalysts. In light of the need for in situ testing of commercial photocatalyst materials, paii labels have been developed that can be applied simply in the field on any surface to be tested, in both a non-reusable and reusable form. One noteworthy application of paiis is where a uniform film has been applied to a photocatalytic surface, and the variation in the rate of colour change across the surface has been monitored and used to generate a surface map of the photoactivity. By this method