This study investigates the effects of water matrices with different natural organic matter (NOM) contents (3.00-6.50 mg DOC/L) and different ozone doses (0.2 and 0.8 mg O-3/mg DOC) on changes in NOM structure and reactivity towards the formation of disinfection by-products. Water matrices differ in the NOM structure and other matrix characteristics (i.e. alkalinity, bromide content) which affect the NOM oxidation mechanism. Ozone treatment results in partial oxidation of the fulvic acid fraction, and increases the hydrophilic fraction (by up to 70%) with increasing ozone dose. A slightly lower degree of oxidability was found in the NOM dominated by the hydrophilic fraction and in water with the highest alkalinity. The residual hydrophilic fraction after ozonation was the most reactive towards trihalomethanes (THMs) and chloropicrin formation, with a higher proportion of brominated THMs in the hydrophilic non-acidic fraction. Bromate formation increases with increasing bromide content in the raw water and with increasing ozone dose, and achieves a significant level for drinking water quality at a dose of 0.8 mg O-3/mg DOC. (C) 2013 Elsevier B.V. All rights reserved.