Since the 1970s it is well known that, though water for human consumption must generally be disinfected before being distributed along the network, the use of chemicals results in the formation of many different Disinfeection By-Products (DBPs). In the case of chlorine-based disinfectants, trihalomethanes (THMs) are the most widely studied: the present work first compares some national and international regulations on this subject, then, in the experimental part, compares the results of a test carried out by disinfecting water of different origin collected in three different Italian regions with different amounts of chlorine. Samples were stored at ambient temperature for seven days, then the determination of THMs was carrried out by Purge and Trap extraction coupled with gas cromatography with Electron Capture Detection (ECD). The result obtained are finally compared and discussed.