The application of ozone in water treatment serves many purposes, such as disinfection and degradation of organic micro-pollutants. To optimise the applied ozone dosage by on-line control, real-time information on the actual Ct value, the bromate and assimilable organic carbon concentration (AOC) is necessary. With UV/Vis spectroscopy, algorithms have been developed for the on-line determination of the Ct-value and the formation of bromate and AOC during ozonation. It was shown that these algorithms allow for the calculation of the optimal ozone dosage and provide a reliable indication of the amount of bromate and AOC formed during ozonation.@en