The concentration of dissolved ozone and Assimilable Organic Carbon (AOC) are important performance parameters in drinking water production. For the measurement of ozone, a spectral algorithm was developed that allows quantification in-situ using a UV/Vis spectrometer probe. Furthermore, a strong correlation between the change in the absorption spectrum after individual treatment steps and the formation or removal of AOC in that treatment step was observed. This allowed the development of a spectral algorithm that predicts AOC formation during ozonation and subsequent removal in further treatment steps. This method has been verified at one pilot plant of the Amsterdam drinking water supply. Includes 12 references, table, figures.