Photocatalyst activity indicator ink (paii) is a substance used to identify the presence of an underlying heterogeneous photocatalyst and to measure its activity. Such inks render visible the activity of photocatalytic coatings applied to various "self-cleaning" products. The inks contain a dyestuff that reacts to ultraviolet radiation in the presence of the photocatalytic agent in the coating. They are applied to the coated product (usually by a pen, brush, or drawdown bar) and show a color change or disappearance when exposed to ultraviolet radiation. The use of a paii based on the dye resazurin forms the basis of an ISO standard test for photocatalytic activity. Applications A photocatalyst activity indicator ink quickly and easily identifies the presence of an underlying heterogeneous photocatalyst and provides a measure of its activity. A heterogeneous photocatalyst is a material that uses absorbed light energy (usually UV) to drive desired reactions that would not otherwise proceed under ambient conditions. Commercial photocatalytic products, which include: architectural glass, ceramic tiles, roof tiles, cement, paint, and fabrics are marketed on their ability to clean their own surfaces (i.e. are self-cleaning) and the ambient air. Paiis address the industry need for a rapid, simple, inexpensive method to demonstrate and assess the activities of the usually thin, invisible to the eye, photocatalytic coatings present on self-cleaning products. A paii, coated onto the surface of a photocatalyst material under test, works via a photoreductive mechanism, in which light absorbed by the photocatalyst drives the reduction of the dye in the paii, thereby producing a striking color change, which can be measured through the use of a simple mobile phone camera + app, in lieu of any sophisticated analytical equipment. Uses of paiis include: (i) laboratory, factory and on-site commercial photocatalyst product quality control (ii) marketing, for the rapid and striking demonstration of the efficacy of the usually invisible and otherwise slow-acting photocatalyst coating, (iii) counterfeit detection and (iv) evaluating new photocatalytic materials. The development and applications for such paiis have been reviewed in detail. Background Heterogeneous photocatalysis is the process that underpins the activity of most architectural materials, such as glass, ceramic tiles, roof tiles, concrete, paint, and fabrics which are promoted as being 'self-cleaning' (or 'air-purifying'). These photocatalytic materials facilitate the oxidative mineralisation of organic and inorganic species by ambient oxygen on their surfaces, rendering the surfaces clean and, usually, hydrophilic. In most commercial photocatalytic products the active layer is