The chlorination of water containing natural organic matter (NOM) and bromide leads to the formation of total trihalomethanes (TTHMs), for which the Italian new regulation on drinking water (Legislative Decree 31/01) indicates a restrictive maximum concentration level of 30 mu g/l. The extent of total THMs (TTHMs) and brominated THMs (THM-Br) depends on many factors: bromide, organic matter, chlorine dose, pH, alkalinity and ammonia concentration. In this study the formation of TTHMs and their speciation was investigated. Different oxidation batch tests with chlorine, chlorine dioxide and ozone were performed on natural surface waters coming from ten different sources (seven artificial lakes and three rivers) used for drinking water production in real potabilisation plants. The results suggest that in TTHMs formation hypochlorite ion reacts effectively with organic matter, and bromide plays an important role in these reactions for THM speciation.