The formation of halo-organic compounds by chlorination of estuarine waters has been investigated under both laboratory and field conditions. Haloforms are readily generated in the laboratory with chlorine doses of 1 to 10 mg/l, the range employed by many coastal power plants. At salinities above 1 g/kg, Br is the principal halogen in the haloform products. On a molar basis, more than 4% of the chlorine was converted to haloforms in some tests. Ozone in the laboratory also generated haloforms in estuarine water; the yields were similar to those obtained from chlorine. However, only traces of haloforms were found in a power plant field site, where apparently haloform-bypassing reactions consume free chlorine much faster than in the laboratory. Identification of these reactions is uncertain, but they may involve formation of stable halogenated macromolecules. A large sewage treatment plant served as a volatile halocarbon source to study the fate of these compounds. The major loss mechanism appears to be volatilization to the atmosphere. Rates for this process are estimated. However, there appears to be some loss under winter ice cover, perhaps because of chemical or biological degradation.