Chlorination by-products (CBPs) are formed as a result of the chlorination of power station cooling water for anti-fouling purposes. Their production was studied at 10 coastal power stations in the UK, France and The Netherlands. Three categories of CBPs were determined: trihalomethanes; haloacetonitriles; and halophenols. Bromoform was the CBP most abundantly present in the efluents of all 10 power stations. At a mean chlorine dosage of 0.5-1.5 mgj/litre (as Cl,) the mean bromoform concentration was 16.32 +_ 2.10 mg/llitre. The CBP found in second highest concentrations was dibromoacetonitrile (DBAN) with mean concentrations of 1.48 k 0.56 pgllitre. Other CBPs detected were dibromochloromethane, bromodichloromethane and 2,4,6-trichlorophenol: concentrations of these three compounds were very low ( < 1 mg/llitre). At those sites at which bromoform was measured in the dispersing efluent plume it was found to behave as a conservative parameter ,‘signtficant direct correlation with plume AT).