Drinking water has to be wholesome and clean, free from chemical and, most of all, microbiological contaminants. The German Landeswasserversorgung (LW), a long distance water supplier, uses surface water and water influenced by surface water as well as groundwater for drinking water production. A combination of several treatment steps including flocculation, sedimentation, ozonation, in-line filtration and, finally, disinfection with chlorine dioxide guarantees a drinking water of perfect quality. Since the water transport through several main and subsidiary pipes of an overall length of about 500 kilometres can take up to four days, the disinfection capacity must be sufficient to prevent bacterial growth. Measurement of the redox potential in addition to measurement of chlorine dioxide concentrations has proved to be a helpful parameter for the surveillance of the disinfection capacity. Many years of experience show that keeping the redox potential above a level of 650 mV guarantees the prevention of bacterial growth and an efficient disinfection.