Seven water treatment works were selected to compare disinfection by-products (DBPs) formed when using chlorination and chloramination. DBPs measured included trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), trihalonitromethane, iodinated THMs and nitrosamines. Generally treatment works that used chloramination were able to meet the European THM regulatory limit of 100 microg L(-1) whereas the chlorinated works found it significantly more difficult. There were no significant differences in the levels of nitrogenous DBPs between the treatment works using chlorination or chloramination with the exception of the nitrosamine N-nitrosodimethylamine (NDMA) which was present at one treatment works in one season.