Ozonation is widely used for drinking water disinfection and increasingly for micropollutant abatement in municipal wastewater. Predicting and identifying transformation products from the ozonation of micropollutants is challenging, due to (i) reactions of ozone (O3) and hydroxyl radicals (OH) with the micropollutants and transformation products, (ii) matrix effects on the O3 and OH chemistry and the generation of oxidation by-products from matrix components, and (iii) limited knowledge of kinetics and mechanisms of ozone reactions with certain functional groups. In this thesis (a) an experimental procedure was developed enabling realistic ozonation in the absence of dissolved organic matter (DOM) and (b) ozonation kinetics and mechanisms of aromatic heterocycles and organic sulfur compounds were elucidated. To achieve realistic ozonation conditions, a synthetic water matrix was constituted with low-molecular-weight compounds (phenol, methanol, acetate, and carbonate) that mimic the chemical interactions of O3 and OH with real water matrices (DOM, alkalinity). This allowed to successfully replicate instantaneous O3 demand, O3 and OH exposures for lake waters and secondary wastewater effluents, for varying temperature and pH. The kinetics and extents of transformation product formation during ozonation of synthetic and real water matrices matched for micropollutant abatement and bromate formation. Furthermore, a significant effect of carbonate on bromate formation was demonstrated in agreement with the theory. Overall, the developed approach allows realistic ozonation studies including O3 and OH, without the constraints of DOM. Second-order rate constants were determined for 19 five-membered aromatic and 40 organic sulfur compounds. Distinct reactivity trends were identified. In aromatic heterocycles, sulfur is completely deactivated. Aromatic heterocycles with 1- and 1,3-substitution exhibit moderate to high reactivities (k_O3=E02-E06/M/s), if C=C-double bonds are available with an increasing trend of reactivity: S < O < N. Benzazoles, lacking a C=C-double bond, exhibit very low reactivity. Sulfides generally react very rapidly with O3 (k_O3=2E06/M/s), though electron-withdrawing groups (k_O3=1-1000/M/s) and ß-lactam structures (k_O3=E03/M/s) decrease the reactivity. Thiols exhibit a much smaller pH-dependent reactivity than previously reported, ranging from k_O3(SH)=104/M/s to k_O3(S-)=E06/M/s. A deactivating effect of adjacent acidic functional groups renders the reactivity of the disulfide cystine pH-dependent (k_O3(pH2)=E02/M/s, k_O3(pH7)=E05/M/s), while other disulfides exhibit pH-independent reactivity (k_O3=2E05/M/s). Further oxidized functional groups like sulfoxides or thiosulfinate esters have low reactivity (k_O3=1-10/M/s), while sulfinates are very susceptible to ozonation (k_O3=2E06/M/s). Mechanistic investigation of the ozonation of azoles revealed small yields of reactive oxygen species (OH, H2O2, and 1O2). 1,3-azoles react initially by a Criegee-type reaction at the C=C-double bond, followed by two reaction-branches, leading to two observed product groups: (1) carboxylates and cyanate; (2) formate, amide and CO2. 2-methylthiazoline reacts by ring-opening and dimerization and finally to N-acetyltaurine. These results enhance the understanding of heterocycle-O3 reactivity and help to predict transformation products formation.