The discovery in 1974 that traces of trihalomethanes (THMs) such as chloroform, bromodichloromethane, dibromochloromethane and bromoform, could be formed during the disinfection of water by free chlorine has led to a review of the entire subject of water disinfection. By appropriate modification of the basic techniques, suitable tests have become available to cover all requirements of works control and field testing, ranging from the simple determination of free available chlorine to the differential analysis of complex mixtures. In test kits for such purposes, convenience in dispensing reagents is very desirable and this has led to the production of reagents in tablet and powder form. The red colors produced in applying the DPD test are matched against permanent color standards, for which purpose suitable comparators are available throughout the world. The DPD range of tablet tests, which forms part of this comprehensive system, relates specifically to the measurement of free available chlorine, combined available chlorine compounds, and other residuals likely to be encountered in the treatment of water by chlorine and related chemicals.