The Black Sea is characterised by an oxic surface water layer and anoxic deep water rich in both hydrogen sulphide and methane, and in between a suboxic zone with very low concentrations of both oxygen and hydrogen sulphide. This makes the Black Sea a useful site for the study of oxidation-reduction reactions in seawater. In this study, the distributions of tetrachloromethane, trichloromethane, 1,1,1,-trichloroethane, dibromomethane, dibromochloromethane and bromodichloromethane across the oxic-anoxic interlace were related to the oxygen/hydrogen sulphide concentrations and thereby the redox potential gradient. All of the investigated halocarbons decrease rapidly in concentration as the oxygen decreases, most likely due to reduction. The results show that the higher the redox potential of a halocarbon, the higher up in the water column, i.e. at a higher oxygen level, reduction occurs. Most of the tetrachloromethane is transformed to trichloromethane as an intermediate product.