The application of ozone in water treatment serves many purposes, such as disinfection, degradation of organic micro-pollutants and oxidation of taste, odour and colour producing compounds. A commonly used method to determine the disinfection capacity of ozonation is calculating the exposure of pathogens to ozone, this capacity is expressed as the Ct value, a product of the residual concentration of the disinfectant (C) and contact time (t). Ozone reacts with natural organic matter (NOM), breaking high molecular weight compounds down into smaller fractions, resulting in an increase in carbon readily available for bacterial uptake. To estimate the regrowth potential of micro-organisms in (non-chlorinated) distribution systems, the assimilable organic carbon (AOC) concentration can be used as an indicator parameter. To optimise the applied ozone dosage by on-line control, real-time information on the actual Ct value and the AOC formation is necessary. Using in process UV/Vis spectroscopy, algorithms have been developed for the on-line determination of the Ct-value and the formation of AOC formation 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 AOC formed.