Contaminated coastal waters can contain nanomolar levels of C1 and C2 halocarbons originating through in situ synthesis from chlorine and through waste discharge. Haloforms are the major volatile products formed from chlorine. In freshwaters, CHCl3 dominates with lesser amounts of CHBr3 and the mixed Cl–Br haloforms; however, above about 5 g · kg−1 salinity, CHBr3 is virtually the only product when typical chlorine doses (10–100 µM) are used. On a molar basis, >4% conversion of chlorine to haloforms was observed in some experiments. Samples from the Back River estuary (Maryland), which receives effluent from a very large urban wastewater treatment plant, contained CH2Cl2, CHCl3, CCl4, CCl2 = CHCl, and CCl2=CCl2 at concentrations exceeding 1 nM. Even when winter ice cover minimized loss by volatilization, downstream mixing was nonconservative, suggesting that chemical or biological degradation processes occur. Under normal circumstances, however, volatilization followed by chemical degradation in the atmosphere is probably the most important loss mechanism. Data on possible biologic effects of these compounds are critically needed.